I would not turn a pump off if the pipe or heat exchanger is where it gets under 40 degrees at any time of the year. You could put a heat exchanger in so you can run glycol in just that part of the system if you have to shut the pump off.
If the only reason to turn the pump off is to minimize heat loss, you could put a mixing valve at the OWB so you can run colder water in the loop when you aren't using heat. Then either a manual bypass or zone valve could be opened to supply hotter water when you need heat.
If you want to save power to, I would use a Grundfos Alpha pump and just restrict the flow till you get 1 or 2 gpm. On auto mode it should cut the power way back.
If you want to do it cheap, you could probably just put a small bypass before the pump and then a valve in the shop. When you open it, the Alpha would ramp up the flow and more would go to/from the OWB. When restricted, the flow should be low enough that most goes through the bypass.
Since that way has no thermostatic control, you would need to monitor it once in a while to make sure if was getting enough heat.