Hi all,
I’m in the process of replacing an old Y-plan central heating/hot water controller/timer and wall-mounted 3-wire thermostat with ZigBee devices. Controller replacement has gone well with a ZigBee relay controlled by schedules in Home Assistant. Wife especially likes the control from her phone! 
The next phase is to replace the thermostat which is an ancient 1980s-looking Honeywell with 3 wires: neutral, live from the ZigBee relay and switched live to the 3-way valve. In my enthusiasm I ordered a line-voltage ZigBee unit with a rotary control knob and colour display from China. Only when it arrived did I realise there’s no permanent live supply in the wall. Of course there’s only power when the controller relay is on! 
Getting any extra live wiring to the wall location would be difficult (and messy) so I started thinking about how to adapt the current wiring. My thinking is the relay and thermostat are just 2 switches in series with the mains power currently going first to the relay and then to the thermostat (when the relay is on). How about swapping that around so the power goes (permanently) first to the thermostat and then to the relay? This would only need a small number of changes in the wiring centre so I think is doable.
Can anyone comment on this idea? My only objection to this I can think of is it leaves non-standard wiring for any future owner - my original objective was to be able to easily revert to a completely standard ‘dumb’ system if needed.
The other possibility is to buy a battery powered thermostat but ZigBee models seem to be few and far between (I’m in the UK) - the principal one available in the usual online stores doesn’t get amazing reviews.
Thanks for reading,
Steve