I have the Hauppauge Colossus 1414. This is an internal PCI card. Hauppauge makes an HD PVR that I think works the same way, but I am not sure. I don't believe it adds much heat to the internals, compared to my video card and cpu. However, I have a large case with good air flow. My HTPC is not near my viewing area, so fan noise isn't much of a concern.
I will PM you my email address and I will try to help anyway I can.
I did write a serial interface a while back, but it really isn't worth much until I can get the information I need out of Media Center. I need away to poll MC (r have MC raise an event0 to find out what channel each STB tuner needs to be tuned to. I found a way to do this when the user changes the channel, but it doesn't work when recording in the background. As with everything, I am sure that adding this functionality to MC is on the list, but I am not sure when it will happen.
In theory, you could use RS232 or HTTP to communicate with the DTV boxes. In fact, I have used both to communicate with my boxes, but not from MC. Right now I have an IR blaster taped to the front of my DTV box so that MC can attempt to change the channel.
I hope this helps,
Alex