I've flown with a few airlines that specifically state that the facilities located in the first/business class cabin are reserved for first/business class patrons.
...as for the likes of Lord Spandex Masher, its hardly elitist. Individuals have paid for a product and expect to receive the benefit of that product, and there are adult undergarments that will help you with your problem.
As for the "takeover" of First/Business class storage space the Cabin Crew on Delta's Manchester/Atlanta flights are wonderful about protecting their premium travelers' space. Smooth as can be, firm but polite "No"s, and directing to the appropriate cabin.