On OS X the default midi driver is Core Audio and since you need to select the CoreMidi driver for a MT32 it doesn't make sense on OS X to have the midi driver option "mt32" available for both Default and CoreAudio.
OTOH CoreMidi is only used for MT32, so it doesn't make sense to have any other driver than MT32 selectable. I *think* the midi effects also do not make sense for a MT32 device.
Unfortunately I couldn't make the patch so that CoreMidi settings for the gump display are in the "#ifdef MACOSX" part, probably could need some looking over.
Fluidsynth and Timidity could probably use a special case as well, since they surely don't need a MT32 driver that sends sysex commands...