I'm thinking that this could be done by using the regular mouse movement statements, just precede them with a command to move the mouse to a known position (like 0,0).
// Move mouse to position (500,250) - 500 pixels from left side, 250 pixels from top
DigiMouse.moveY(-2000,-1000); // Move cursor left 2000 pixels (some number greater than the width of the screen) and up 1000 pixels (again, some number greater than the height of the screen - This will place the cursor at a known position (0,0) since you cannot move the cursor beyond the screen.
DigiMouse.moveY(500,250); // Move position to desired place on screen
Now, I have not tested this, but looking things over, it seems that it should work.