I'm not sure what kind of bridge you're talking about exactly with the HomePNA but OpenBSD or any of the BSDs out there should be able to bridge 2 networks with little difficulties. I'm not sure if there are drivers for HomePNA cards out there for the BSDs but it's worth looking into.
Linux has bridging code but last time I tried to use it things didn't work out so well (I was trying to make a bridging firewall and it failed with linux whereas it pretty much works with OpenBSD

).
Gaidin