heya shreddr, good questions as always. I only play a machinist on TV, I am actually...well let's not go into that right now. I see from your pics and Ralfs comments that all the R1200 telelever arms are made from the same cast aluminum parts. From the looks of it they had allowed for some fairly heavy modifications at the factory as well.
The conical bolting of the ball joint imposes some complication to the adjustable slot approach to varying the steering head angle. However, you could have a machinist do a stepped slot and make some conical eccentric spacers something like this to change the ball joint position.
Something like this with a mating half to fill the slot:
