I've never personally tried it over FW, but AFAIK, any TCP/IP based network should work.
That said, I don't know what you're trying to test, but on a 100 base network, I've never "overloaded" a host with only a few users. I use RDP to XP & Vista VMs. VNC is ok, but the screen refresh is not fast enough for me in some cases. If you have a gigabit network, then I seriously doubt there'd be that much noticable overhead. In fact, the kind of website testing I often have to do is to see what happens when there's too much traffic. We want to know if the end user is experiencing too much delay. So in this case the overhead is not necessarily a bad thing. And all this on a very basic P4/2GHz, 2 GB of RAM, SATA RAID1 to run 3 VMs.
If you still are worried about bandwidth, what about just adding a 2nd NIC? Gigabit NICs and unmanaged gigabit switches aren't that expensive and fairly common. Cheaper and more common than a FW hub anyway. So unless you already have FW in all your workstations and a FW hub and cables lying around....
Anyway, that's just a few things off the top of my head.
