Well, you could "probably" pull it off. An HD Homerun should find the QAM signal whenever it does a scan. But how would you make MCE change the channel on your Cable Box/Sat Box without also changing the channel that MCE is displaying? Seems like you would have better luck buying one of those multi thousand dollar professional HDMI capture cards that can handle HDCP and using that for video input, and then using an IR Blaster to change the channel for you.
I have read a few posts where people have mentioned stuff like that. There is a company called Blackmagic that makes an HDMI capture card, but it isnt HDCP and I think that the end of the thread talked about no vista support as a capture card through MCE.
If there was a way to capture the HDMI output of a cable/sat box, and control the box via RS-232 or Firewire I'd be all over it, but as many times as someone has asked the question, no one has had a solution.