not sure the value of the idea. seems neat but surely response time would still be sluggish if they are all parked at the nearest hospital, unless they plan on parking these on random people's rooftops or are "on patrol". both of which would have pratical and legal issues currently.
in the UK at least most "at risk" areas tend to have defib boxes about, usually in old phone boxes etc. greater awareness/availability of these may be more useful.