I managed to build a custom SDL library (from libSDL 1.2 sources) that exposes the source of mouse & keyboard events. People interested in how I did that will find the patch there: patch. It uses XI2/RawEvents. It’s very hackish and ugly, but it does the job. Note that it probably breaks stuffs useless to the sixaxis emulator.
This allows to use several mice and keyboards with the sixaxis emulator, which is a nice feature to play split-screen FPS games with friends