diop I don't know of one but you could build one yourself using an old PC, known-good OS, and one of the known-good graphics cards. Even the old known-good NVS 295 supports H.264 I think. But the PS3 should, too? I have Movian installed on my custom firmware PS3 Slim. It plays many videos, including 264, but not 265, and has network functionality. I'm not sure what you mean by converting. But Movian plays almost everything you throw at it.
It may also be worth to take a look at ARM boards, since they seem to have a variety of GPUs. I haven't had luck with Raspberry Pi (3 and below), but maybe there's something else that is usable.