I used this DIY on my other M3...Mazda 3...auto tranny. It worked fast and I got to completely flush the tranny fluid.
I wonder if the pump on the DCT is accessible and you are able to remove the hoses like the DIY above. Then maybe you could use the DCT pump to drain the old fluid while pumping in new fluid. So the transmission never goes dry and you should be able to do a complete flush.