I have an older SqueezeBox (version 1), which doesn't provide the bridging. That's a newer feature, and it seems like a stretch to buy another one ($300?) just to have bridging. That, and I really dislike the SqueezeBox GUI.

New thought: I was thinking that iTunes "sharing" might be the answer. I tried playing local MP3s from a laptop through the AirPort Express and it basically didn't work -- lots of skips and jumps. If that doesn't work, then there's no hope of doing via iTunes sharing, which only adds more wireless traffic.

EDIT: I also tried just playing from upstairs to downstairs (i.e., only taking one wireless hop). It certainly worked better, but I was still getting a non-trivial number of skips (maybe one or two per minute). It's just bad enough that I'm probably going to have to give up on this. The proper answer, once I'm in a house that I own again, is to run a normal Ethernet wire.

(And, I was also careful to put my home network on a different channel from all of my neighbors in an attempt to minimize interference.)

*sigh*