I had a similar idea for a DRO mount: a plate mounted to the (threaded) lube-hole for the read-head, and a slit-and-thread clamp to mount one end of the scale/rail to the TS quill.
Because of the length of the scale, it was a bit too wobbly, so I started looking for some way to make a clamp. I found some square channel in my stock that was a perfect size for the TS: used a bandsaw to make cutouts for the TS lever and the front leg of the base, drilled a hole to screw it to that front TS lube-hole. I haven't mounted a DRO to it yet (screwed up the quill clamp and have that in the mill for repair), but have been using the lathe with the square-channel mount in place and it's surprisingly sturdy for something held on by one screw and wishful thinking (and a LOT of filing-just-so).
Maybe some day I'll take a picture.