I know several people have swapped out the Y or X axis to add the option for a rotary axis with success. But a plug and play option, maybe even make a QCW option where a section of the table can be removed and the rotary axis drops in it’s place. You could even make a way to zero the center of the axis similar to the way the current touch probe works.
Honestly, I’d be happy with a rotary even if it wasn’t that easy to set up. If it’s already 5-axis it should at least be able to have a setup without having to swap the x or y.
It would be very straightforward to set up with Masso, the only unknown at this point is what Onefinity will be offering as far as their electronics box and its available connections.
Even then, adding your own version of a rotary only requires control signal wires from the Masso ‘A’ axis (that is the only one available for the added axis) to a driver and stepper motor that will run the rotary system of your choosing. The number/type of these connections will depend on your chosen driver. The only other thing left is a PSU for the driver, which should share the same VDC- as the controller.