It should be possible to have constant surface speed without spindle feedback (turning mode).
If I consider that the real speed is equal to the spindle speed set up by mach, CSS should work.

In fact there is an option in the ports&pins -> spindle setup -> special functions

that have: Use spindle feedback in sync modes

If it is unchecked, it should do exactly what I mean, but it does not work, still expects the feedback.
Also this should be very easy to implement.
Filipe