Have you thought about using a circular print platform to utilize the arm's full range of motion?

I am currently building a repstrap on a similar principle, driven from a shaft within a pipe.  The big challenge would be to get rid of play in the mechanism, and to get high enough resolution in the outer edges of the build area.

When I was thinking about this, I looked at two stacked turntables with the extruder moving up and down. The turntable bearings can be printed, either tapered roller style, or sliding bearings. I was able to make both types turn reliably with me standing on them!

Interesting idea however the filament feed would need to be looked at as the joint for the arm is concentric with the output arm. Additional spacing would be required to extend the hotend from the arm nadding more complexity to the kinematics.