File format plugin API for Spotify?

At Spotify, we have one and a half day of ‘hackday’ ('code whatever you want’, the '20% time’ concept from Google) every sprint. Since the past few months have been a bit hectic finishing up the new 0.5.0 release, we haven’t really had time for that in a while, and thus compensated by having a whole week of crazy hacking this week. I managed to finish two features in that time; one in the iOS client (of which I will only say: Loren Brichter is my hero!), and one in the desktop client: file format plugins.

Last year, one of my hackday projects was integrating blargg’s Game Music Emu into Spotify, thus letting you play NES, GameBoy, SNES, Genesis, PC Engine and other console’s music files in Spotify. Just a few days of work, and suddenly I had the best console music player app in the world, with playqueue, playlists and whatnot. However, GME is LGPL and we statically link our dependencies, so there was no way I could release it :(

Thus, I aimed to hit two birds with one stone: by adding a file format plugin API to Spotify, I could a) enable anyone to add support for their favorite file format in Spotify (flac, mikmod, sid, …) b) let me release the GME support as an open source plugin!

However, I’ve never designed a plugin API in C before, and if we’re going to release it and support it for quite a while, it should be well designed, easy to use, and cover all the basic needs. It’s versioned, so it should be simple to add new things to the API, but changing the existing ones might be tricky. So, my question is: do you have any feedback for me on this api?