Update - swapped my fluid:
I found an independent BMW shop to swap my M-DCT fluid. Rumor has it that the units total dry capacity is ~9.5l. They were only about to get between 4-5l out of mine and they claimed they used the computer to cycle the pumps in the transmission (no idea if they actually did). My plan is to swap again in 10k miles or so. Those swaps together will remove about 75% of the total contamination. The proper fluid was not at all tough for them to locate. They also said the fluid that came out looked practically new. Since all it does is bathe the gears that is not too surprising. Sure wish there was a way to get all of the fluid out. That might also require some priming of the various hydraulic systems in the unit, however that capability must be somehow built in. Job took less than 1 hour and I was charged 1 hour of labor.