I have a Hauppage card in my PC, which has a "pause mode". The pause file is accessible by XBMC, via Samba, so it does stream almost live TV to the Xbox this way.
The Hauppage card only supports recording one channel at a time, so live TV cannot stream different channels to different Xboxes with such a setup. However, once programs are recorded (on the PC HD) different Xboxes should be able to access them via the LAN. <