It IS possible, but I would highly advise against it. I think 20" is a bit big for a Z4, and would look imbalanced compared to 19" front wheels. You also should consider potential rubbing issues, since you are on H&R springs.
I've never noticed the rear wheels appearing smaller in a staggered setup. As long as the tire sidewalls are the same height, then the overall look of the wheel should stay the same. If you run taller sidewalls on the rear tires, then they will appear to look smaller by comparison
235/35/R19 front
265/30/R19 rear
This would be an ideal tire setup. If you go 20" rear, then you'll have to get 265/25/R20.