Mouse Move to Keypress

People with a rare condition called Progressive Nuclear Palsy can't look down or have the necessary dexterity press and release keyboard keys but they can use their whole hand to move a large trackball. So I'd like to request a feature where a mouse pointer move can be mapped to a key press. This would allow moving the mouse to simulate the pressing of arrow keys so the person could move down a list of selectable items like emails or browse through photos.

On a MacBook the trackpad would work as normal allowing an assistant to move the mouse pointer as normal to open and app and activate a first responder that can accept arrow key input, e.g. the email list in Mail. With a trackball connected, upon the move of the trackball, BetterTouchTool could capture the pointer and hide it and detect a move gesture and simulate an arrow key press. At the end of the gesture the pointer can appear again. If the trackball is moved and stopped but it hasn't moved significantly enough (a configurable parameter) then the gesture is cancelled and would need to start from zero the next time, which would prevent against accidental small movements triggering presses.

Also I'd be grateful if anyone has any suggestions of alternative software that could provide this functionally, thanks.