I've been diddling about with my Sieg X3, and it's quite nice to have X and Y axis DRO. The kicker is the Z axis, where there are TWO sources, first, the Quill, second, the Z axis handwheel.
My thoughts- use a mag base to attach a dial indicator to the bottom of the head, the main support sticks straight down. Set the other arm so it is parallel to it. Now slap on a dial indicator and indicate from the base of the mill, not the table....
Downside, you will have about 1" of measurement. But in milling, that should be OK. Upside, it's a non-drill solution where you don't have to worry if it's perfectly aligned. Most likely cheaper.
Looking for a frugal electronic dial indicator with SPC output. That way, I can cobble the head travel onto the DRO display, and just have the quill travel on the scale.