I had a go at just randomly adding some dirt anyway: good old galaxy brush!
The result is... meh. It doesn't really look all that great in game, but I guess at least I have a non-pristine alternative.
Edit: found some splattery brushes, now it looks a bit better.
Further edit: and in game, where I still think they look a bit too subtle. I used the same overlay for both, because why not etc.
Yet another edit: eventually realised I just needed to desaturate and lighten the base texture in order for it to work. My PC eventually makes an appearance modelling the original outfit: