I have finally started working on this mount again. I got an old Quick-Set Hercules 5302 tripod from a friend. It is rated for 150lbd but I am betting it will hold a bit more.
I took another piece of the scrap 17-4 stainless and machine a spud that adapts the 1/2-13 threaded hole in the center of the mount to the 1-1/4" spud on the tripod column. A set screw in the spud locks in a steel socket in the column.
The OnStep controller never really worked how I hoped so I have decided to go with a SiTech controller. A couple of days ago my SiTech Servo 1 controller showed up. First thing I am going to try is making a 2nd order filter to attach to the motor outputs of the controller to smooth out the PWM and then scale it to the +/-10v analog input the servo drives have. Then the encoder feedback from the drives will connect to the encoder inputs on the SiTech. If that does not work I will use the SiTech to drive a couple small dc servo motors to and then use another encoder to drive the Mitsubishi servo drives through their pulse inputs, they can be configured to take quadrature signal inputs as well as CW/CCW and step/dir modes.
I used LTSpice to design a filter, we will see how well it actually works out. I put the controller on the o-scope and found it runs at a PWM freq of 23.45khz.