I would love to see something like this come to fruition. The fact that you already have an established userbase is great and I'm positive that you'll see loads of MediaPortal users jump on board. We can show those Tivo owners who's smarter!
I'm in the process of wrapping up our new platform/API (about a week out). Once that is done, I'll will be turning my attention to making this a reality. I've grabbed the SDK for MP to do some light reading in the meantime.