How can I get the same element ids that the DeleteStaleTemplateCaches Task ends up with before the element has saved?
This is not much of an answer but basically Brad was right - I just needed to replicate the logic that the DeleteStaleTemplateCachesTask does in its getTotalSteps() and runStep() methods.

This allows you to work out exactly what caches will be just about to be deleted. I'm not going to post the code here as it is literally a copy/paste from the DeleteStaleTemplateCachesTask.