We talked a few weeks ago about this and he is standing on the same ground. It cool, but very gray. You don't want to push the man (ATF) or tangle with LE, if you know what I mean. In fact, one of our customers, who happens to be a very large collector of machine-guns, went off the handle. He was stating that the paperwork that one gets back clearly states that the NFA weapon must reside at the address stated in the paperwork and in the possession and control of its owner. No exceptions. Definitely an even stronger position then what I have. And also noticed on assaultweb.com (the post you put up) was also mixed. So, I think we opened a major can of worms.
I took it upon myself to contact ATF and get a definitive answer on this issue. Only problem is that they don't take email with technical questions and it had to be done via snail mail. It going to be like that commercial, "not going anywhere for a while!". In other words, it gonna take time. We may have an answer back by Xmas if we are lucky.
I'll post either way when we get an answer.
Talked to an attorney on the Fourth issue and he kind of on your position and that there is protection. But, if they suspect foul play, lets say narcotics or some other illegal activity, they will pretty much do things by the book and do what they want to do. Kind of spooky the was he explained how the judicial and LE work hand in hand. More like, what Fourth! But the bottom line was, there is nothing to fear. They are not coming knocking.