I think I know the answer, but want to confirm.  I have an eForm pending for an 870P SBS.  My plan is to engrave just the short barrel in a conspicuous place and leave the receiver as-is.  I only plan to have the one short barrel and may regularly swap back and forth back to the 18.5" barrel.  I want to avoid engraving the receiver in the event of a future sale.  If I do decide to buy other short barrels, I have no problem engraving those as well but I don't expect that to happen.  

Any reason this wouldn't be kosher?

That works fine. Only thing required to be on the frame/receiver is the serial number. All other required markings can be on the barrel.

I thought I had read (in the context of FN Scars), that although engraving on the short barrel was proper, you couldn't then change back and forth to the longer barrel, because when the longer barrel is on, you have a registered SBR (although not currently in that configuration), that doesn't have engraving on it.  Maybe it's different for SBRs than SBSs.  Maybe the other info I read was just wrong.  Maybe I misunderstood the prior threads.  Any follow up education would be helpful.  Thanks.

If it's not configured as an NFA item, it's under the purview of the NFA regulations. You can put a long barrel on a SBR/SBS and sell it to Joe Blow as a GCA firearm.

It just has to be marked any time it's configured as an NFA item.