I think I see where you're coming from. I originally configured my UFH and radiator system without hydraulic separation of any kind. The question of starving the radiators is a bit academic when the UFH return warms up in only 7 minutes and I never noticed a problem with the smart pumps conflicting. It was Y plan, no bypass was needed, and it worked happily.
I did, briefly, experiment with a setup similar to yours. I set the flow across the bypass to a fixed rate to bring the DT across the flow and return, with the UFH circuit in a heated state, to around the same as the design DT for the radiators. I can see that a tapstat could be used to maintain a set return temperature no matter whether the UFH circuit is hot or cold. I have seen solid fuel (wood) boiler with what must be an essentially similar setup, although I think the one I saw had electronic rather than mechanical thermostatic control. The flow is short-circuited until the boiler is at 60° and only then does flow to the emitter gradually start.
I did, briefly, experiment with a setup similar to yours. I set the flow across the bypass to a fixed rate to bring the DT across the flow and return, with the UFH circuit in a heated state, to around the same as the design DT for the radiators. I can see that a tapstat could be used to maintain a set return temperature no matter whether the UFH circuit is hot or cold. I have seen solid fuel (wood) boiler with what must be an essentially similar setup, although I think the one I saw had electronic rather than mechanical thermostatic control. The flow is short-circuited until the boiler is at 60° and only then does flow to the emitter gradually start.