I've always heard that all stripped receivers must by law be marked as "other." Still, it doesn't hurt to make sure. Just look at the form before he calls it in.
As far as I know, once you put a stock on it for any reason, or even just a buffer tube with the ability to take a stock, you've just a built a rifle and that receiver is forever more a rifle. I've never heard that you can go back and forth under any circumstances. I wouldn't be trying that, though, regardless of whether there's actually a loophole or not. It just leaves too much room for confusion, and too much opportunity for some critter at the ATF to try and make a name for himself.