"how about a player that had the usual play pause stop buttons and that you could interrupt at any point with a click of a button of key and then enter the name you wanted for the file split off?"
That looks to be the best scenario. Simple and efficient (maybe if possible add some kind of a visual display of the midi data in the file so to be able also to spot silences between pieces in a long midi file and split that way... but that can be more tricky no...? )
Best to you
Alain