It's not just about NOTAMs
There are also physical differences between an instrument runway and a non instrument runway, in terms of baseline widths for the approach surveys and approach gradients, not to mention the required runway and runway strip widths for an ALA are much narrower than for a registered or certified aerodrome. Mos 139 Part 1 table 7.1-1 refers. In addition protection of the PANs-Ops surfaces is a requirement for a certified aerodrome, not so much for an ALA. So when you are in the soup at a certified aerodrome you are pretty much guaranteed that nothing unexpected, like a windfarm or an NBN tower will have been installed without assessment. My conversation with CASA was that yes, GPS would allow precision approaches to paddocks eventually - IF all the other standards for an instrument runway are met.