How about the ability to lock or manually set the XYZ dimensions? For the former, select which dimension(s) you wish to lock whenever it reaches that value and it will ignore further mouse movement in that/those direction(s). For the latter, enter the desired dimensions and pick a starting point, the preview will snap to the closest corner to which you move your mouse.
I hardly think creating an entirely new tool for this is necessary, but expanding the capabilities of what we already have should be amenable to most...