The country has already setup a Mutual Aid plan that they've required all agencies to ratify. The agreement is an official document, and I will try to lay my hands on it again, but to summarize it states that you agree to be part of the County Mutual Aid plan. There is also the CEMP - Country Emergency Management Plan, which you can get somewhere on westchestergov.com, and lists the capabilities of each agency that has subscribed to the mutual aid agreement, but doesn't get into the nitty-gritty planning of what will happen if X happens in Hastings, for example, who will be dispatched and how. How is still a problem in that not all agencies in the County are dispatched by 60 Control.
I am in two minds as to whether having all Fire and EMS dispatched by 60 control is a good thing or a bad thing. Of course I would like Emergency Medical Dispatch to be able to triage the BLS calls and to determine when a Paramedic is really needed.
In your suggestion, it would only work in a paid system, where people had to 'do the right thing' and respond appropriately, but with an all volunteer system, staffing, commitment, and relationships can all be problematic.
ICS-100 does dictate that the closest available units respond. In a multi-alarm or MCI, it would be to a staging area that is close to the incident. For the most part, the respond to quarters to cover xxx units is an appropriate response to this level of incident command -- but never when it puts the responding units geographic area at risk.
-Just some thoughts.