Jabawocky, the Airservices proposal for ADS-B relies on line of sight UHF coverage. If the lady in question could not get through to air traffic control on VHF, it is very likely that she would not be within ADS-B coverage.
The problem is that once you have hills and mountains, a system that relies on a direct line of sight radio link to a ground based transmitter often has limited coverage at low levels – probably the very place you would need protection.
Also, ADS-B for GA aircraft sounds to me to be a very expensive way of providing a service to protect pilots from an incident as described with XMV. I think we should concentrate on getting the maximum level of VHF radio coverage – by both ATC and Flightwatch – before we spend extra hundreds of millions of dollars in providing a duplicated service with ADS-B.
Remember, when the FAA goes to ADS-B in 2020, it is keeping the full secondary surveillance radar coverage above FL180 across the entire continent. If Airservices does the same thing (even just to cover the J-curve above FL180), the funding for the low level ADS-B will not be available.