I was not able to tune the spindle
Just looks like the Ultra 5000 has the wrong parameters for tuning a servo. Can't get it hold position and be stiff at the same time. Either can get it position tuned or be stiff. Put in an AMT programmable encoder instead of the original 1000 ppr one the other day. Set it to 2048 ppr and tried to tune it. Had only less than an hour to play with it, but it didn't feel any better. In fact may be even worse, but I am not sure. The belt, although pretty tight, starts behaving as a musical instrument string when I try to increase the position gain. And in the same time I get overshoot on position. The point when I am happy with the position gain and there is no overshoot, the spindle loses stiffness completely.
Regarding the plugin, yes, Greg sent me a version with quadrature signal for the spindle, but he said he didn't think it would solve the problem we were experiencing, but it would be nice if it did in some way. If you like to play with I can send it to you. It works very well here, and in spite of what Greg said I haven't been able yet to get any symptoms of the problem we were seeing with the spindle. SRO seems to be working fine as well as CSS. But I only had a few minutes to play with it.