How about all the bodyguards have an emergency status button thing? (I forget what it's called). That would allow for the police to be aware that the body guards or the mayor him/her self is in danger. I also think that the mayor's bodyguards could have their own communications system like /com (maybe /mo, /com1, or /ncmo).
This way the mayor's bodyguards can realisticaly communicate, but at the same time not be a part of the PD or interfere with/listen to the PD's communications. If assistance from the PD is required then they could use the emergency status button, then the PD would be know that there's...well, an emergency. The best thing to do with the emergency button thingy is to have all the PD officers know exactly
who, the mayor's bodyguards are. That way if we see the emergency status alarm go off we would automaticaly know that the mayor is in trouble.
I'm not even a bodyguard
, I be SWAT.