I have seen "crowd scene" brushes in the asset store, I would say you could make one that uses your own custom people shapes and add the jitter, but you'd still have to add and adjust details as necessary. Maybe that's close enough for now?

I do that all the time for long distance crowds- in fact I had to clean out a bunch of old crowd brushes recently because I had too many. I’m taking crowds of distinct individuals (or, say scales, books, foods, rocks, trees, etc) that I’ve drawn and need to fill.