I've seen this done before, so I will suggest you put a preview page on your site that has black and white gradient bars with a short paragraph about display brightness and contrast adjustments for proper viewing of your galleries. Maybe your clients will adjust their system for optimal viewing. Or maybe they won't, but at least you've done your best to explain to them that your images are print ready and properly exposed and processed.
I always reference Norman Korens website:
Monitor calibration and gamma
Although this is written for a photographer and not a customer.