Couldn't you simply overwrite the idealloadshvac availability schedule with your ems? Also, to avoid HVAC convergence problems, you'll need to make sure sudden opening and closing of windows is not going to be problematic.
Finally, the AFN has a hybrid ventilation mode that you can run out of the box. I think the AFN ventilation to control temperatures as EN15251 Adaptive model is also supported. Be aware that EN15251 Adaptive model and its reporting only apply if the outdoor temperature is greater than about 10 to 15 C, depending on level and this may be misleading.
There is also a human comfort controller object which will try and emulate when a user will open or close a window based on thermal comfort. It assumes there are window contacts to turn off the HVAC from memory.
What I find is true in winter times is that users under ventilate to the extent that windows could be assumed closed as a simplification. The exception is when users leave the space for 15 to 30 minutes on a regular bases and open the windows during this time. This is common practice in residence and school classes. Often energy is wasted because the heaters and thermostats are left on and there is no sensor control on the windows. Also, temperature wise this is a bit like driving where you get stopped a lot by red traffic lights.
Keep us posted how you get on.
Jean