Hey Hanafuda,
I don't mind working on MA900s, I actually just made one detachable and pseudo balanced tonight. The wiring is actually pretty simple, the resistors are built into the boards connected to the back of each driver, so the wires from the cable are simply soldered to the board as they are in other headphones.
I borrowed this picture that
@sunneebear posted back in 2015 to label the soldering points because his photo is so clear (hopefully he doesn't mind).
If you wanted to go balanced you would have to desolder the wires from locations "signal to right driver" and "ground to right driver" from the left driver. The board on the left driver bridges the ground from the cable to the right driver's ground and it also connects it to the ground of the left driver. The "right signal from cable" connects to the board only to bridge it to the thinner cable that runs through the headband to the right driver, it doesn't do anything more than that on the left driver. Beyerdynamic does the same thing with their DT line. To go balanced you would desolder the thinner cable that goes through the headband from this board and connect it to your right signal and right negative from your balanced cable. You would solder the left signal to the left signal soldering point labeled in blue in the picture and the left negative to the ground labeled in dark grey in the picture (or the black location, it really doesn't matter).
If you want to DIY it and have questions, shoot me a PM or contact me on Etsy as I'm not on Head-fi as much nowadays.