I found a way to improve the sweep of the rear blade..
The pitch angle of the motor itself can't really
be adjusted to alter the angle of the shaft where it emerges out of the bootlid, since it's screwed in to captive nuts.. You can try and twist it slightly or pack the screws with washers to artificially tilt it.. but that's a bit of a bodge really.
On the other hand, the wiper arm itself can be tweaked. The two separate parts of the wiper frame sit inside the recess of the wiper arm body... and the blade carrier rests against the frames. The blade carrier angle is "fixed" by the points where the carrier touches the frames.
I worked out that if you scored a notch into the blade carrier, where it touches the lower frame, the blade carrier would become more angled (as it can rotate further round, so the top of the blade moves closer to the glass.
I also enlarged the angle where the wiper arm halves are joined together with the plastic pin. They are both "square" cut.. so would touch when the blade carrier rotated slightly. By cutting in a small angle into the plastic, it allows the top part of the arm to rotate further round too.
It's not 100% perfect now.. but the arm clears to about 1" from the top of the arc. A bit more tweaking should sort it..