Apple TV (1st generation)

[failsandwich]

The 1st-gen Apple TV has been pretty much left for dead by Apple. iTunes' update servers don't connect to it anymore, and it doesn't support logging in/syncing music with modern iTunes either. To use it, you have to either sync it with an old computer or install CFW such as aTV Flash or OSMC. 
So, all in all I think RockBox would be a pretty good fit for it. It is basically a glorified iPod, after all. It wouldn't be that difficult to port Rockbox to it, especially considering it runs on an x86 Pentium M CPU and doesn't have any particularly unusual or specialized hardware, unlike most MP3 players.

[saratoga]

If you can install Linux on it then you can already run the SDL application. Probably would need to map remote control buttons but otherwise I would expect it to mostly work.