So, Stuart's comment is still valid. I will stop looking for a USB solution, and instead see if I can find a low power chassis with a PCI slot. While more expensive, it is probably money well invested.
It might be worth buying/reusing a standalone access point (perhaps reflashing a linksys router or something) and connecting via ethernet cable. That way you have more flexibility in choosing locations: ie, the OpenBSD router can be somewhere that's convenient for interfacing with your ISP's equipment, and the wap can be somewhere with good reception. 'course, then you have two boxes to deal with, so it's a trade off.
