Reminds me the nightmare I had when I flashed the coolant from my 92 400E(M119) 1 year ago.
It took me 6 hours!! I had to use the car the next morning so I almost fell into sleep underneath the car at 2 AM, I was virtually soaked with coolant, made me want to kill the guy who designed the cooling system 3 times.
There is no trick, you definitely need an u-joint, and a lot of patience and brutal force.
Would it be OK to drain from the radiator and left drain plug, then refill the cooling system with just water, run until warm, and then drain it again, refilling with the proper coolant / water mix?
This sounds like a good idea if you do if many times, at least much much... easier than draining it from engine.