Thanks Basher for digging that out.
From that thread it sounds like if I configure the Nest in on/off mode for hot water, and in OpenTherm mode for central heating, I might be able to get it working, so long as the Ideal Logic+ system boiler can sensibly handle switched live (on/off) and OpenTherm inputs at the same time.
Regarding 'sensibly handle' multiple inputs, the questions to understand I think are what happens if:
a) Nest is calling for heat via switched live and not via OpenTherm. I assume the boiler will prioritise switched live at that point and fire on the flow temperature configured in the boiler.
b) Nest is calling for heat via OpenTherm but not via switched live. I assume the boiler will then fire on the temperature requested via OpenTherm.
c) Nest is calling for heat via both OpenTherm and switched live at the same time. I assume that in this case switched live will be prioritised and the boiler will fire on the flow temperature configured in the boiler.
d) Nest is not calling for heat via either interface. I assume boiler will stay off as expected in this case.
I've asked Ideal support to clarify how their Ideal Logic+ system boiler specifically will handle these four scenarios and will update here if I get useful input from them.
@hometech thanks for pointing out the DYI option with the raspberry pis. It sounds like a fun project but I'm still looking for a more accessible setup so it can be serviced by most heating engineers without my input. Else my house will have to come with its own GitHub repository if I ever decide to sell it...