We have a constant problem of guys not wearing all of their PPE, not wearing it correctly, and (of course) not wearing their seatbelts. The officer and driver need to take control of this, and (more importantly) departments should clearly state in their policies and training that if you aren't wearing PPE (including a seatbelt) then the department will not be fully liable for your bills if you are injured.
I dislike the idea of any sort of seatbelt interlock system in an emergency vehicle. With sensors, logic relays, wiring and all those connections, there are too many things to potentially go wrong. I can just see it now- "Well, I guess we'll have to call another department for their ladder, guys. I don't know what's wrong...we're all buckled up, right?"
It's a matter of human accountability. It shouldn't be left to some goof-proof devices to make us do what we're supposed to do.