@jimp:
So yes, it can work, but it's likely going to be slow (even far, FAR more so with Untangle)
there, I fixed it. ;)
You can hack the sysctls to disable filtering entirely for if_bridge which will make it pretty quick, not nearly as quick as a switch though, you're going to be limited by the PCI bus using a standard PC. If you don't care about getting a lot of throughput between the hosts it won't matter.