I'm still very impressed with the Rockbox MIDI playback engine -- I've tried a number of relatively complex MIDI's, and only the very most complex caused the Rockbox MIDI to glitch (buffer overrun or some such message).
I did some more experiments, and the problem with the volume control is that it maxes out at too low a level -- i.e., I couldn't hear the MIDI playing on the airplane even at the highest volume level, while if I cranked my mp3's to nearly the highest level, I could hear them.
Perhaps the MIDI player volume control needs to be able to go beyond 100% -- I've noticed that a number of software MIDI players on PC's can crank up the volume beyond 100%. Of course, these MIDI players can also globally override the speed, so one can multiply the song beats-per-minute by a fudge factor.
The problem with MIDI files is that for classical pieces, the piece is broken up into different movements, which are typically separate MIDI files. So if a long piece is broken up into 5-10 movements, it would be nice to play them all together without having to restart each one separately.