Can you pan the MIDI instruments to one output all the other tracks to the other? (we're half way home)
If yes, then there are still a number of other hurdles to clear.
Do you have a re-amping box to put between the M-Box output and the relatively high impedance guitar amp/Talkbox?
The Talkbox will have to be mic'ed back into the recording, so you'll have to monitor all this panned hard L/R through headphones.
On a side-note: I don't know what kind of talkboxes you've been looking at, but the Rocktron Banshee is a self-contained, self-powered talkbox that does not need a separate guitar amp to drive it, so it might be the most versatile for your situation.
A) the Banshee might be more forgiving on hot/cold incoming signal levels, (specs on the Banshee say Max. Input +9.5dBu and Input Impedance 1MΩ) and I read somewhere the output of the M-Box mini is only +6dBu max. You probably weren't going for a clean tone anyway, so you might find a tolerable setting.
B) the abrupt make/break switch used with other original design talk-boxes is notoriously hard on guitar amps.
*Bonus info: If you buy the Banshee go to the hardware store and buy your own tubing, we will not live long enough to allow the tube they provide to off-gas the foul chemical smell it emits - I don't know what they were thinking. I kept mine around for siphoning gas should there ever be an emergency, in which case the taste of a mouthful of gasoline would be preferable.