I'm thinking of using the MIDI phantom power feature of my MP-1 (which
will require a mod to the controller).

Irritatingly, this uses pins 6 and 7, which means that a 7 pin cables 
is
needed, even though standard MIDI leaves two pins unused.

I'd like to use the 5 pin cable I already have.

I looked into this, and evidently, the reason might be that some 
devices
can accept a reverse MIDI stream using the two unused pins out of the 
five.

It should be a very easy mod to jumper pin 6 and 7 to 1 and 3.