Toilet limescale remover does improve the clean to some extent, but with hard-water marks, they're almost impossible to remove completely; though I've never tried a Mineral Deposit Remover.
Call me a cowboy, but I generally inform the customer of the problem, so they don't think I'm doing a shoddy job, and on future cleans I just do my best (which is to clean the window, leaving the hard-water marks where they are).