yes, basically exactly how your 406 handles the zone valves and circulation pumps already.
I know the Taco pumps can handle 0 flow for short periods of time, but I was unsure what kind of pump you were using. If they can handle it, then you're probably good to go.
As far as the relay... It's drastic overkill for just simulating a thermostat. You would hook the coil up to your tekmar controller. You might have to do some voltage adjustment as it requires 24v DC to actuate. Hook one leg of your "thermostat" wire to pins 13 or 14, then the other leg to pin 5 or 8, being the compliment of 13 or 14. Did that make sense?