Try using
sketchto access the underlying PApplet object! Let me know how you go.
This worked after a fashion for arcball, but context was wrong:-
In JRubyArt, propane etc (and vanilla processing) the Applet instance is used to initialise many libraries methods that use java reflection to access :pre, :draw, :mouse_pressed etc typically in setup so they pick up sketch context (size, render_mode etc). But like in the case of arcball they might do a translate as well.
Unfortunately the above arcball sketch renders somewhat oddly instead having a box to rotate with an arcball I get box image on flat surface, which does apparently rotate (mouse_dragged) and zoom (mousewheel)