Please what you mean with “works great for every other program except dosbox”. What does another program, but does not mDosbox?
I succeed with disabling the black cursor on nvidia wireless controller controlled with right joystick. With physical mouse I was not successful, seems this is android limitation and depends on manufacturer. I don’t own external touchpad so I can’t test it now. But if touchpad simulates touchscreen then probably it cant’t be hidden. As I wrote I succeed hide black cursor controlled with gamepad. It’s because you can tell system that mouse event was handled and processed. But this is happening for touchscreen the same way and cursor wont hide;(