the main problem with unichrome drivers is that they still don't support MPEG4 acceleration. On the unichrome mailing list I've read that a new release with MPEG4 support is not that far to come but as to now it is not possible to say when.
They currently support MPEG2 hw decoding, and according to what I've read around they are quite efficient. On a epia M10K it should be possible to play at the same time 2 MPEG2 streams with up to 70% CPU load in total, that sounds quite good.
I've also read about epia ME6000 with MythTv, so probably you could give your 500 MHz a chance ...
Anyway I agree with you that it would be the case to rebuild kernel and some key packages with epia patch/support.
As an example, on my M10K board mp3 are played fine with alsaplayer, but pretty poorly with xine, and this is probably due to some lack of optimization (and possibly lack of knowledge on how to configure it the proper way ...)
So in general there are surely some adjustment/optimization in order to have Pluto running at 100% on a epia board, either as MD or hybrid.
The point is that I'm also afraid that setting up a epia box with all the proper kernel and whatever else may result in some broken functionality in Pluto, or may lead to some strange surprise on the first apt-get upgrade.
It would be great if someone at Plutohome could give us some hints on this, just to give us a starting point for some tests.
Radu, if your're reading this please say something ...