After another trip to the garage and some tinkering, I think the center diff will be the way to go. I'll need to figure out dog bones to finalize it. That should be the only thing I need to order once I figure out which to order.
Here's what it would look like from the top - the red spots are where I would have to cut out the chassis to accomodate the spur (about .5cm on each side).
Here's a view facing to the front of the MGT:
And one facing to the rear:
I'll need to make an aluminum mount to hold the diff braces, then another to hold the motor.
The only dog bones I have laying around are the OFNA ones in 83mm and 92mm. With the 92mm one set up in the rear, I would need a 117mm dog bone for the front. Essentially, I have 210mm-ish to work with for two dog bones.
I found OFNA part 36110 that's 88.45mm and HPI part 86058 that's 123mm. I think the two of them would work together and since they're both sets of two I'd have a couple extras.
While I figure out the further details of the center diff I'll still use my modified MGT tranny and see how that works out :)