Boilers do not use the information provided in the "Room setpoint" message. They operate based on the "Control Setpoint" and some other bits and pieces. So changing MsgID 16 will have absolutely no effect.
The job of regulating the room temperature lies with the thermostat. If a thermostat doesn't support the remote override room setpoint (like the Toon), then there is nothing you can do on the opentherm interface (short of completely implementing your own room temperature control algorithm).
To change the room setpoint on the Toon from Domoticz, you will have to use the methods provided by Eneco (via their web portal), or
hack the Toon.