I would assume, that the WPC5 in combination with the ISG web does provide the same modbus interface as my LWZ504e - give or take some values. See the following compatibility overview.
Configuring the hot water target temperature is not possible, because it was implemented as a HVAC device and not as a water heater device.
I am not able to further support the Stiebel Eltron integration, because my ISG web is communicating with the SMA Sunny Manager - which disables modbus on the ISG web completely. Having both enabled is not supported according to the Stiebel Eltron support. It would be nice, if someone is interested to take over the integration.
Due to the modbus issue, I looked into Node-RED and requesting values directly from the ISG web interface and stream it via MQTT broker to home-assistant. It actually works quite well, and I assume changing values via Node-RED would be possible as well.
@jeanbart: Yes, ISG web has the possibility to enable the user/password access. However, I don’t know if the user is/can somehow be used for modbus.