it certainly would also apply if the aircraft was observed to be 'significantly' below the glide-path.
Absolutely. In my experience, however, self-positioning tend to result more often in high/fast approaches than (dangerously) low ones, as pilots (occasionally) seize the opportunity to make a more expeditious arrival. So that (more frequent) situation was what most of my post was alluding to.
Our (aircrew) notion of a 'de-stabilised' approach in a modern, swept-wing aircraft tends to be one where excess height and/or speed cannot be lost prior to touchdown - a low approach can always be improved by adding power; if high/fast and at idle thrust already, one cannot reduce power any further so the situation becomes more critical.
We (our company) can only self-position when in receipt of radar monitoring (unless fully visual), so in that regard we would always expect terrain and traffic advice, as usual - the only difference being that we are doing the nav and speed control.