Correct me if I'm wrong but....
It's my understanding that any FFL can be in possession of, or service, an NFA item. I am reasonably certain of this. I'm not an SOT or anything other than a Type 1 FFL and we've taken in suppressors and SBR's, etc.. from LE, the Feds, and privately owned for years. BATFE has not said one thing to us about it and I have gone through an audit(s).
Now, fitting up barrels that are below the legal length. That's a little more dicey when you don't actually have the paperwork done yet. For that we test fire on a pistol reciever (AR stuff) and we never ship a barrel assembled on the action or the lower receiver.
Assembled with no paper, you play with fire. Disassembled no one cares from the legal perspective. It's just junk on a bench.
Don't take my word as gospel. I'm no "sea lawyer", but it might be worth researching a little more as I don't think this is nearly as big a deal as you might think.
Good luck.