Notes to today's revision of CAAP166-1 are interesting:
Note 1: The intent is to ensure broadcasts are made on a frequency that other aircraft in the vicinity will be monitoring. A broadcast made only on Multicom 126.7 at an aerodrome whose existence is only known through local knowledge is misguided, as transiting aircraft relying on aeronautical charts for aerodrome location information will be monitoring area frequency.
Note 2: Nonetheless, pilots should study all authorised aeronautical charts relevant to their flight route and category, in accordance with CAR 233 (1)(h). This is because some details may be omitted from charts of different scales even though they cover the same area (usually to avoid clutter).4
So the answer to a previous question about differences between charts seems to be if it appears on
any relevant chart, then it's the CTAF or 126.7, only if it doesn't appear on any relevant chart should we use area.
Seems to me to make some sense, particularly given that Farmer Brown has no mandatory requirement to broadcast anything at all at his farm strip, shouldn't be a lot of clutter on area.