Hi Aaron,
Saw your post on thegreenbutton forum. Glad to hear you're interested in looking at integration to Media Center. I like Media Portal just fine, but the wife is another story!
I searched for other remote control plugins, but have come up dry. Hopefully that doesn't mean that it's ridiculously difficult to develop for - but with MS, i guess you never know.
However, I did find a couple of pages on msdn outlining how media center handles input.
I hope these prove helpful:
Understanding Mouse, Keyboard, and Remote Control Input Retrieving Infrared Commands
Please let me know if there's anything I can do to help.
Thank you,
Ryan