Just closing the loop on this topic. I've gotten my Delta B3 Ethercat X, Y, and Z servos working properly over Ethercat and I decided to take the cheap and easy solution to spindle control for now. I picked up a USB to RS485 stick quick inexpensively and just this afternoon got it working with LinuxCNC over MB2HAL. It took a fair bit of trial and error to figure out all the syntax, but I got there in the end.
Long term, I'd like to find a solution that would allow for full real-time control over the spindle servo for rigid tapping, but I'm plenty happy with this stopgap solution for the time being. Now that I have all 4 servos running properly on a bench, I'm going to hook everything up to my mill tomorrow.
