I don't think its physically possible to get in there and do this, but, I offer it as a possibility:
If you can use the Dremel and put a notch on the O.D. of the cap screw and then setup with a cold chisel and a 5 lb. sledge, the combination of shock from the sledge and the instantaneous massive torque developed will probably break the screw loose.
I recently had the displeasure of doing the exact same thing on one of the nuts that secured the stabilizer bar bushing. When looking at the nut, there were no flats whatsoever. In fact, at the top of the nut, the threads of the bolt were poking through.
But, I managed to get a chisel on the side of the nut and got it to bite. I heated the nut as hot as I could get with Mapp gas and then started pounding. Took about 30 shots with the hammer...........additional heat.............and some Kroil..............but..................by the grace of the Mercedes gods..............it broke loose. This was really dangerous because the head of the stud is inside the frame rail and I have absolutely no idea how to replace such a thing.
A photo of this nut is below. The tiny land for the chisel is on the right.