This was attempted, but did not work properly. From my internal notes:
Unity (at least the version I'm using) goes pretty wonky when Mac full-screen kicks in, reporting incorrect mouse positions. I have not yet found any way to work around this, other than to disable full screen.
However I have made the window resizeable, and now neatly allow any scaling (while still snapping to even multiples of 1.0 when you get close).
Not ideal, I know, but until Unity fixes their mouse-positioning bugs (or we manage to ditch Unity), I think we're stuck with it.