I don't understand the reasoning of the article you reference. The evotouch collects the heat demand of all zones and controls the boiler based on that. If all zone valves are closed, an opentherm controller could still request the boiler to heat. But so could an on/off controller. So how does that make on/off control better?
Or is the on/off controller supposed to be connected in series with a relay contact of the zone valves? So that if all valves are closed, the signal from the thermostat cannot get to the boiler. If that's the case, I guess a new feature could be made for the OTGW that would allow the valves to be connected to a GPIO pin. If all valves are closed, it could switch off the boiler. Similar to CS=5, CH=0, or something along those lines.
I don't understand the configuration well enough to be able to imagine what would be needed. But in any case, it is possible to connect an on/off thermostat to the OTGW and have it still talk opentherm to the boiler. See the (updated)
description on the web site. I haven't used that option myself. So I can't really say how well (or poorly) it works in practice.