LinuxCNC as ELS for lathe?

Basically, the ELS requires a way to set the TPI / mm pitch.
You can certainly do this with LinuxCNC, and the fact that LinuxCNC has all the drivers might be a good enough reason.

I have always fancied doing an ELS with BCD switches (eg www.ebay.co.uk/itm/125300438656 ) and a metric/imperial switch. That way you could run the Pi without a display and run a non-gui non-g-code HAL-only config.

I would probably write a custom component for the job, rather than glue together all the required existing modules, but it certainly could be done with standard HAL modules.

I think three digits is probably enough for most purposes? 1 to 999 TPI and 0.01 to 9.99 mm pitch.
Then you would need 4 IO pins per digit + units. so 13 inputs. Unfortunately the 7i96 only has 11 inputs, so this scheme would maybe have to use a scale of 0.01 to 3.99 mm and 1 to 399 TPI.
(though you could use the Pi built-in GPIO for the inputs instead of / as well as the 7i96.