Yeah, the trick is you need to emphasize a certain type of low-contrast feature without emphasizing anything else. If there was a way to detect only perfect circles, that would be a huge gain.
That might be a good question to ask of the G’MIC folks. If there’s a way to even somewhat isolate and enhance dust spots, maybe a reference guide image could be made with (only) dust spots emphasized. I played a bit with G’MIC but I’m pretty hopeless and came up with nothing useful so far.