I really love the clever mechanism for calibrating the Z offset - print something and then probe the printed object. It would be cool if the microswitch were "omnidirectional" - could print a small (e.g. 1 cm^2) cube with each tool and measure the top and all 4 sides using the probe. This would measure the size of each cube and the location of the center of each cube.
plenty of touch probe designs out there. would not be too hard to make a "tool" that is a touch probe, or even a temporary tool used only for calibration. that is manually removed. Not sure how useful that info is though. calipers are cheaper and easier to use.
Strongly disagree that calipers are easier to use... the point would be to accurately calibrate the X, Y, and Z offset of every extruder tool with one click.