What's a 'quadrangle rule'? After 40 odd years in ATC I never heard of it.
As for using ASR; this would not necessarily prevent a pilot from inadvertantly entering CAS when operating below a CTA so I would think pilots would have to get an actual QNH, possibly from the nearest airfield, as they presently MUST do when operating below a TMA (FIR might be able to help but would they have the closest local value available?).