A while back someone mentioned that they would like a spindle lock on the SX3. I too thought that it would be a nice improvement. I was mainly interested in being able to lock the spindle for scratch engraving. I wasn't so much wanting a brake that could be used to slow the spindle to a stop. I think that could be better done with dynamic braking and would require a relay or two.
Following are a couple of pictures of what I have done and one drawing.
I had mentioned earlier the desire for a spindle brake but for tool changes, I am not to found of the spanner wrench on the bottom of the spindle. My goal is to be able to do a tool change with on tool or less. I am working on setting up a pneumatic draw bar system. It looks like you system of pinching off on the belt will work fine for your needs, please report in if you run into any wear or marking on the belt.