What about this approach. In 3DS Max and I'm going to assume in other 3D programs like Blender, when you create a shape, a box for instance, the first click places the corner of the object. When you let go you drag out the base, click again then drag out the height, click again to finalize the shape. Can a similar approach be used in the 3D view. In 2D you drag the size of the exclusion zone and it defaults to full height. But if you want to specify a height, you go into 3D view and use the above suggested approach.
I know, I'm a pain in the butt, but as a daily end user and not a programmer it can be nice to have input about various things that a dev may not consider at programming time.
And STILL absolutely loving Repetier, the cost was soooo worth it and has made administering multiple printers much much easier for me.