Shouldn't the TAF, METAR, area QNH and current QNH from the aerodrome sensor all be within 5 HPa? Otherwise there should be amended TAF/area QNH and/or SPECI, any of which would be directed/broadcast by ATC to IFR/VFR. Admittedly in decades of passing amended TAF's to pilots, not one has ever asked for the QNH's (or temperatures). We also religiously pass area QNH's to aircraft on descent to G, whether they use it or not I don't know.
If the minima is designed to allow area QNH to be used it must have a fair margin built in. I've seen area QNH's with a 5HP split on different sides of the same aerodrome!