Lots of places probably would...

But it would be pointless, as engraving a removable trigger guard doesn't meet the engraving requirements. Nothing stopping you from installing one on every SBR you own, but they'd also need to be marked on the frame, receiver, barrel or pistol slide (if applicable).

As mentioned above:

§ 479.102 How must firearms be identified?
(a) You, as a manufacturer, importer, or maker of a firearm, must legibly identify the firearm as follows:

(1) By engraving, casting, stamping (impressing), or otherwise conspicuously placing or causing to be engraved, cast, stamped (impressed) or placed on the frame or receiver thereof an individual serial number. The serial number must be placed in a manner not susceptible of being readily obliterated, altered, or removed, and must not duplicate any serial number placed by you on any other firearm. For firearms manufactured, imported, or made on and after January 30, 2002, the engraving, casting, or stamping (impressing) of the serial number must be to a minimum depth of .003 inch and in a print size no smaller than 1/16 inch; and

(2) By engraving, casting, stamping (impressing), or otherwise conspicuously placing or causing to be engraved, cast, stamped (impressed), or placed on the frame, receiver, or barrel thereof certain additional information. This information must be placed in a manner not susceptible of being readily obliterated, altered or removed. For firearms manufactured, imported, or made on and after January 30, 2002, the engraving, casting, or stamping (impressing) of this information must be to a minimum depth of .003 inch. The additional information includes:

(i) The model, if such designation has been made;

(ii) The caliber or gauge;

(iii) Your name (or recognized abbreviation) and also, when applicable, the name of the foreign manufacturer or maker;

(iv) In the case of a domestically made firearm, the city and State (or recognized abbreviation thereof) where you as the manufacturer maintain your place of business, or where you, as the maker, made the firearm; and

(v) In the case of an imported firearm, the name of the country in which it was manufactured and the city and State (or recognized abbreviation thereof) where you as the importer maintain your place of business. For additional requirements relating to imported firearms, see Customs regulations at 19 CFR part 134.

Pistol slide was added in ATF Ruling 2013-3, as pistol-based SBRs weren't really a consideration when the regulations were written.

Yup.

I've got 14 sub-16" AR uppers. $$$$$$

In addition to the cost of engraving all fourteen, many are old, collectible Colt variants where any additional engraving would really hurt their value. I still enjoy shooting them occasionally on my three AR SBRs and Colt M16A1, though.

...because the regulation says it is.

You can still shoot a firearm without a trigger guard...but it's kind of hard to shoot one without the barrel.

CFR = Code of Federal Regulations

27 CFR 479.102 How must firearms be identified?
(a) You, as a manufacturer, importer, or maker of a firearm, must legibly identify the firearm as follows:

(1) By engraving, casting, stamping (impressing), or otherwise conspicuously placing or causing to be engraved, cast, stamped (impressed) or placed on the frame or receiver thereof an individual serial number. The serial number must be placed in a manner not susceptible of being readily obliterated, altered, or removed, and must not duplicate any serial number placed by you on any other firearm. For firearms manufactured, imported, or made on and after January 30, 2002, the engraving, casting, or stamping (impressing) of the serial number must be to a minimum depth of .003 inch and in a print size no smaller than 1/16 inch; and

(2) By engraving, casting, stamping (impressing), or otherwise conspicuously placing or causing to be engraved, cast, stamped (impressed), or placed on the frame, receiver, or barrel thereof certain additional information. This information must be placed in a manner not susceptible of being readily obliterated, altered or removed. For firearms manufactured, imported, or made on and after January 30, 2002, the engraving, casting, or stamping (impressing) of this information must be to a minimum depth of .003 inch. The additional information includes:

(i) The model, if such designation has been made;

(ii) The caliber or gauge;

(iii) Your name (or recognized abbreviation) and also, when applicable, the name of the foreign manufacturer or maker;

(iv) In the case of a domestically made firearm, the city and State (or recognized abbreviation thereof) where you as the manufacturer maintain your place of business, or where you, as the maker, made the firearm; and

(v) In the case of an imported firearm, the name of the country in which it was manufactured and the city and State (or recognized abbreviation thereof) where you as the importer maintain your place of business. For additional requirements relating to imported firearms, see Customs regulations at 19 CFR part 134.

But you can't shoot it without a barrel. You can shoot it without a trigger guard, just like you can shoot one without a handguard.

I don't know what you're arguing. The regulation has been posted several times. Acceptable locations are the frame, receiver, and barrel. The ATF added "pistol slide" to the list in 2013.

FWIW...the current Form 1 includes this in Instruction (2)(j):

(7) Markings: The maker is required to mark the firearm with his or her name, city and state. All markings are to be in compliance with 27 CFR 478.92 and 479.102. and that's the regulation I posted above

So, follow the regulation, or don't. It's up to you. Just know that Instruction 9 on the form says:

9. Penalties. Any person who violates or fails to comply with any of the requirements of the NFA shall, upon conviction, be fined not more than $10,000 or be imprisoned for not more than 10 years, or both. Any firearm involved in a violation of the NFA shall be subject to seizure and forfeiture. It is unlawful for any person to make or cause the making of a false entry on any application or record required by the NFA knowing such entry to be false. (and not that I'd expect it to happen, but the penalty can actually be increased to $250,000)

Whoa...hold up. If we're going to have to discuss which regulations are stupid (or illogical), then we're in for a long thread. As far as I'm concerned they're all stupid, not to mention unconstitutional.

That being said, this one isn't that stupid. The reason is without a barrel, you can't have a short barreled rifle. Remove the barrel, and you know longer have an NFA firearm. Being serialized has nothing to do with it. Heck, go look at most of your shotguns...and you'll find gauge, city & state, and other info on the barrel instead of on the receiver.

Remove the trigger guard, and you still have a (functional) short barreled rifle, which falls under the purview of the NFA.

Yes, you can remove the barrel and the marking are no longer on the firearm, but that's fine. If you put a different barrel on it that's at least 16" (and results in an OAL of at least 26"), you still don't have an NFA firearm...even if the registered lower is attached. But, if you do put a different short barrel on it, you're back to having an SBR...and that firearm must be properly marked. So, those that don't engrave the lower must engrave every short upper they plan to use.