> Is there some way to determine that the mouse button is being held
> down, not simply a single MouseDown Event?

Sure: return True from MouseDown, and now you’ll get repeated MouseDrag calls as long as the mouse is down. (After which, you’ll get a MouseUp event.)

> I would like to repeat code constantly in the MouseDown Event as long as it is being held down.

Simple move it into MouseDrag, and return True in MouseDown.