It's obvious you WANT to take the atf side, but if you go to their website and read the comments below, you will gain some ACTUAL info of what's going on. In particular this is one comment.
http://airsoftoutletnw.com/
Ben Martin 2010-02-25 14:22
we've had gunsmiths try to make a WE TTI lower work with a real steel upper, and it is true that a AR trigger pack will sit in a WE lower, but the trigger mechanism is inoperable with the WE lower unless you heavily modify the body. these are not readily convertible as per customs argument.
Either way you want to look at it, it's still an overreacting, over reaching government agency. I suggest you spend some time on their web site, there are many people posting there from all over the country with a lot of expertise with these. The gist is you could do it,, but you'd need pretty much need to modify ALL the parts and not just the lowers.