You have to be careful with wifi; unlike an ethernet switch, bandwidth is shared between all wireless cards.
IIRC Snapstream once offered a WMV/BTV client box for around $99, but that role was eventually superceded by BTV Link and MPEG hardware encoders.
You can, however, recycle an older P2, P3, or Athlon box for this role to act as a sort of "client HTPC" for your other TV. With decent video, sound, and network cards; a copy of BTV Link; and a FireFly remote it should work. Of course with this solution you get the full BTV viewscape interface and there is no annoying MPEG-to-WMV transcoding time.
Of course, if you configure showsqueeze on your existing box and if it's fast enough, you could probably just get one of those Windows Media receivers for your TV.
As far as watching BTV on multiple TVs, well, I don't use TVs much anymore. Computers do so much more. With BTV Link on my PC, I can put TV in a window and work with email, read news headlines, write up a paper, etc. while listening for the commercials to end. I can also watch my recorded shows just fine.
I just wish my wifi connection wouldn't give out so easily. That's probably due to having the PC across the house from the access point and simply using the crappy default antenna.