I also share the concern to make them public.
However... I think the request to have them public is perfectly understandable. The documentation to fluid_player_set_player_callback is as follows:
* Change the MIDI callback function. This is usually set to
* fluid_synth_handle_midi_event, but can optionally be changed
* to a user-defined function instead, for intercepting all MIDI
* messages sent to the synth. You can also use a midi router as
* the callback function to modify the MIDI messages before sending
* them to the synth.
So the intended use-case (intercepting TEXT events for a karaoke player) seems to be in scope for this callback. We even have public functions/macros available to get information about an event, like its type for example. But if a user wants to actually interpret the returned information, he needs to recreate fluidsynth internals in his own code. That is a little inconsistent, in my opinion.