Yes you could do this, it does seem like a bit of a farf though and needs manual intervention. Do remember that if you are keeping one rad as a bypass then it can't have valves that can be turned off by mistake.If I didn't want the rad to heat when the hot water is turned on in the summer then I could isolate the kitchen rad (turn down TRV, would I have to turn lockshield off also?) and the flow would be redirected to the hallway rad, therefore keeping the bypass open?
The most correct way would be to install an autobypass valve into the system and then take the feeds to those rads from the normal CH system flow and returns, then none would need to be on in the summer.