So Josh and Eric identified a USB 2.0-based 802.11a/b/g adapter which has soft MAC support in the Linux kernel. So, we thought we'd get one or two. Turns out they're hard to get then we thought.
The part we want is the Senao Networks, Inc. NUB-8301. It's also called the SUB-8301 in certain countries. We can't find it in the US, apparently.
We originally tried to get it from a UK company, Solwise, but that ended up in disaster: they don't accept international credit card orders, we spent $40 on an international wire transfer and the lost it when their routing number turned out to be wrong.
Sanao is represented in the US by EnGenius Technologies Inc. (1580 Scenic Ave. Costa Mesa, CA 92626, 714.432.8668). I left a message with Brian (x860).