Well truth is I only know of one FILTER that can do stuff off the GPU, and it's called FFT3DGPU().
It's a denoiser, so it cleans up grainy video to make it look like new again. Problem with this filter is that it's very demanding CPU-wise, so much in fact that encoding is slowed down by the filter.
I encode an XviD (no filters) and it goes 80FPS; if I put this denoiser, it goes to 10FPS. So the filter bottlenecks the whole encoding process. That's pretty much the idea.
Furthermore, it's the only one of it's kind, as far as I know, but because I do SO MUCH video encoding, it's worth my consideration... Probably not what you were hoping for, but there you have it.
Cheers and thx for the suggestion,