There is no such animal. It doesn't matter how the information is sourced, a METAR is a METAR is a METAR. Even if ATC give you the "automet Bulla at 0530", how do you know that that QNH wasn't fed into the system at 0523?
Personally, I don't see why a time limit of say 30 minutes couldn't be applied to allow use of the METAR QNH to use the standard minimums. The "-100ft" prohibition is because even though a METAR may be issued at say 2200, the QNH may well have been taken 10 minutes before that, chewing into the 15 minutes.
AUTO METARS cannot be used for actual QNH, just forecast terminal QNH minima
No, a METAR QNH cannot be used for
anything.
Forecasts are really only nothing more than a "guess" about what the QRH will be in the future and we all know how inaccurate the BOM's forecasting can be at times!!
The time groups leave a big gap with respect to changes. Surely a METAR could be used with some common sense reading of the Trend over the last couple of hours?
No, if the actual QNHs were drifting outside the issued TAF, the BOM would revise the TAF.