WRT54G is a series of routers, there are several models with different hardware.. as for it not being on the supported product page, why would it be? it's a piece of hardware inside an embedded device..
The chipset might be something by Broadcom or Atheros.. but you didn't give us a model to work with so how can we possibly know for sure?
Testing for compatibility is simple.. boot OpenBSD.. if it's supported, you'll find out right away.