Glass scales for DRO are relatively inexpensive. The Bridgeport EZ-Trak I bought at work uses them in the servo system. So getting static accuracy is probably not too difficult, but the dynamic accuracy is a whole different can of worms.
I can’t remember who wrote the white paper, but it described “rounding a square”, and measuring how much tangential material was left over at the flats, as a test of dynamic accuracy.
The scales will measure the axis position. But when you move the table over to the other end of travel, the shifting weight tilts the knee, and now your cuts that were supposed to be parallel are not. None of the scales measure this rotation.
Then, as the machine warms up, differential thermal expansion starts to twist things further...
I can’t remember who wrote the white paper, but it described “rounding a square”, and measuring how much tangential material was left over at the flats, as a test of dynamic accuracy.