well, the way that the JK Labs boxes work (and also the nbox, for that matter), is that the active cables go straight to the box, and the mic bodies get skipped altogether. Because of that, you've also got to figure out a way to power the mic capsules. Tim's JK Labs box for the MG's just acts as "bodies", in that, you need to feed the JK Labs box phantom power, and then the box powers the capsules. The nbox, and some other JK Labs boxes, are battery powered, and provide power to the mic caps directly.
I know that Nick (the creator of the nbox), is working on a similar one for MG caps (and AKG, I think). I don't know how much time he's already put in, or how much longer it'll be until we can buy MG nbox's, but just something to think about. It certainly can be done, and I agree with you that just about all the electronic components are pretty standard stuff that one could buy from a catalog (the exception being the mounting thing that the caps actually screw onto, as you noted).
So, to sum up, in addition to providing some sort of amplification so that the signal can be sent down a wire, the box that the active cables attach to also has to be able to somehow provide the optimum voltage to the capsules. as to what the optimum voltage is for the MG's, I'm not sure. but it is definitely something else that must be done and considered in the design. Please let us know how it all goes...