OK, so to sorta answer my own question, I was able to fit two rather large bookcases in the xB. One was about 79" tall and 15" wide, and the other was the same height, but about twice as wide.
I pulled the front passenger seat all the way forward, laid its back down flat, and removed its headrest and the headrest from the seat behind it (which was down, of course). Both bookcases fit straight in, on their sides, though they did obstruct my view out the front passenger window, so merging into the right lane was pretty much impossible. But it worked.
[[obvious]]So, yeah, there's lots of room.[[/obvious]]
'05 MT BCP xB