I can somewhat understand where this is coming from all though I'm not sure that there is a role as the game currently is. Traditionally, i the US, the states will activate the National Guard in times of disasters. NG units traditionally back fill or fill roles such as large area search and rescue, transportation of people out of affected areas in military vehicles, helicopter rescues/search and rescue. There are also other types of missions that active guard units perform especially in the world of WMD (Civil Support Teams and CERF-P teams come to mind) as well. I personally have multiple CST WMD teams across my game that I use in the role of HM/Foam units. I can see where you could add another layer of units similar to the Coast Guard units you already have (Create a National Guard base, allow it to just build search and rescue units, add a helicopter extension {maybe allow CG and NG air units to satisfy each other air missions and/or EMS patient transport}, and add a HM extension for mass decon, WMD events etc).
Just my 2 cents......