Ken, I don't want to be a nay sayer but you may want to rethink your plan.
As you may know in my shop the DC and "auto" blastgates are triggered by devices that work by the same principle as the Sears autoswitch. I designed my system so the blast gate for every machine is activated that way, EXCEPT the miter saw.
The miter saw is often only on for a brief amount of time, depending on how wide a board you are cutting. More often I make repetitive, quick, 2-3 second cuts. The end result with an autoswitch is that you would be cycling the vac on and off rapidly- really tough on a motor. Also, the vac might not come up to full suction fast enough and likely would not be on long enough to collect and clear all the dust generated out of the piping. Even if automatically turned on, you would want the vac to stay on for 15 - 20 seconds after stopping the saw, to clear out the piping and so more of the residual airborne dust is collected.
Just something to consider.
