I found this picture:
And got an idea.
What if the AR was an AR pistol with a 10.5 inch barrel and sig brace (no stock and over 26" with the right buffer tube) and you could attach a shotgun as short as a serbu super shorty, but since it is attached to the AR it is over 26". Hmm? Does anybody see any reason that would be illegal? I can see that it may be nessasery to somehow permantly attach it, but heck, the birds head is not "permanent" either. There used to be a kit for attaching a shotgun to an AR, it was in "Predator", with that kit the shotgun had no grip, so I guess a shotgun without a grip is "non functional" and thus non measured untill it is assembled? So maybe if it was removed from the rail without a grip it would be OK?
DISCLAIMER: I am not planning on building anything like this, I am mearely interested in seeing if a company could/would produce it legally and sell them (if legal).