Late to the conversation, but is there any reason any electric ball valve with manual override couldn't be used?
The manual override should just be a "Knob" on the top that rotates with actuation right?
Thinking something like this: http://www.oscsys.com/store/product/291
Then figure out a way to couple a rotary encoder to the manual knob to sense the real world position of the valve? http://www.oscsys.com/store/product/1001689
Parsing the output from the encoder should be a lot easier than all the crap you need to do to make a 0-10v transition with the controller.
But then again, I'm just guessing