Howdy Farcome.
The latest Jepp, 25 April and AIP amendment covers CCs' question.
There is no change to the "actual QNH" requirement for the 100' minima reduction on an approach. It's just a separate clause now.
Your reference to AWS sources for this 100' reduction applies to an AWIB. (ie) broadcasts of actual weather conditions.
I think what CC is asking is refering specifically to a metar and the use of it under the new amendment, for a QNH source to set at the IAF of an approach(not for the 100' reduction).
Metars can be derived from either an approved observer or an AWS so they can be used for this purpose, as i said before, under part a) of Jepp Terminal clause 5.3.1 (same for AIP)
Hope I'm not giving a 8um steer with all this but is sounds right to me.
Barge