All right, so I've just gotten off of the phone with a guy at the Barnes and Noble in Tysons (McLean, VA). /Generally/, the policy is to only allow items bought in-store to be signed. But author's preference will factor into it as well. The guy said probably the best way to go would be to buy one item in-store, and then say, "Oh, I've also got one/a couple more to get signed, as well," depending on the author and their preference. So, yeah, bring along a few books, but I would expect to have to go to the end of the line after 1-2 of them, depending on the line size.
I know that for my part, I'll probably be preordering a copy of GS, because I want to get a copy for the local library as well. And taking along a couple Codex books.
Also, if there are any other locations folks would like me to call to ask about signing policy, I'll be more than happy to do so. Course, Pris may hear differently from Jim on some of it, so of course there's that to bear in mind.