MediaPortal hasn't been designed for "media extenders". For them to work with MP, they would need to have their own little MP client written into the firmware of the device. Definitely possible, just not very likely.
Thin clients aren't really suitable for front ends for MediaPortal, mainly because they offer very little in the way of video acceleration. They are after all designed for running office apps, not full screen video
I don't know how XP Unlimited works, but I can't imagine it would be suitable for serving up the MP interface and full screen video.