I wonder whether otherwise you could alter the behaviour of the boiler
by arranging a zone valve and bypass in effect across the flow and
return of the CH circuits - i.e. to open when any of the CH zone
valves open.

Not quite sure I get you. Do you mean mix some flow water into the return so
fooling the boiler to modulate down?

To be honest, I'm surprised boiler manufacturers don't have facilities to
control the flow temperature programmatically. It seems a common
requirement, particularly for underfloor heating and now that condensing
technology promotes lower radiator temperatures (but still requires nice hot
water in the cylinder).

Christian.