Try contacting some of these piston manufacturers. Not sure if any have M42/M44 pistons in stock, but I'm sure they'll custom make anything you have in mind. These guys are all top-notch forged piston makers, commonly used in the BMW forced-induction world.
CP Pistons Ross Pistons FYI, Ross lists custom 4-cyl pistons for BMW's at less than $500/set.
Wiseco Pistons I believe Top End does not actually make their own, but rather has JE Pistons make them:
JE Pistons Those Ross pistons are a bargain compared to the TopEnd/JE ones, and probably better quality.
Constant