Exactly -- the N9's GPU is the N900's GPU, on a smaller die and with a higher clock. It's still the same GPU.
Isn't that sort of thing already possible? I've been using a Samsung HDTV's USB input to play HD video from my N900 for some time now. In any case, using the phone as an external hard drive to a TV doesn't use the GPU at all, and so doesn't suffer the kind of problems with HD that playing files locally does.