@Alistair3149: does GETSHORTDESC work? It doesn't seem to be an available hook even when SHORTDESC is?
Topic on Extension talk:ShortDescription/Flow
{{GETSHORTDESC:}}
acts like a magic word. You can put it in the page and it will retrieve the description.
https://www.mediawiki.org/wiki/Extension:ShortDescription#Retrive_short_description_on_wikipage
@Alistair3149 Hmm, are projects like testwiki and enwiki that allow for the 'shortdesc' parser magic word not really using this entire extension - and that's why getshortdesc doesn't work?
They are not using this extension. On WMF wikis the functionality is provided by Wikibase