I did. The main problem was that it only takes a file path, not a file
handle. I looked at whether it would be easy to add a function for this
but it didn't seem to fit into the playlist design.
True, the fluid MIDI file player doesn't extend to anything other than files (by name). But looking at Mix_LoadMUS (in SDL_mixer), which I assume you are extending, most of the music types pass char* file (the filename) along to the music format loaders. Can't you do the same to fluid?
If you do want to load data from a non-file (such as an SDL RWop), it seems you'd just need to replace the functionality of fluid_midi_file_get_division and fluid_midi_file_load_tracks (the only operations performed on the midifile object itself by fluid_player_load in midi/fluid_midi.c) -- not actually deal with interpreting the track data itself (you wouldn't need to know the semantics of each individual event).