I would not add electric baseboard since they are a step in the WRONG direction.
Remember the goal is vacuum insulated wall panels with a R value over 200 and a ground source heat pump.
Stepping back to reality, you would be better off metering what heat is used or simply splitting the heating price in half.
Having them pay for heat only helps if they have some ability to control how much heat is used.
1. can the tenants change the room temperature or use a set-back thermostat?
2. Can the tenants seal up air leaks or otherwise reduce heating costs?
If not, then you are probably better off finding ways to reduce heating costs for them since they pay for it in the end anyway as part of the rent.
Edit: I am assuming you have variable water temp system and the tenants can adjust the water flow rate to keep from being roasted or frozen.