In what ways can solid egg yolk from a boiled egg be processed/preserved?
I'll add an answer that doesn't address your question directly, but does address the underlying concern:

If patrons regularly discard part of the dish, you should ask why. With hard boiled eggs cooked in a large scale cafeteria kitchen it's easy to overcook the eggs, just leaving them in the water for hours on end. This results in a greyish, crumbly and unappealing yolk, often discarded.

If you can get the kitchen to improve their process, perhaps with smarter egg boiling appliance, the end result will be more appealing and thus less wasteful.

This, of course, is extremely optimistic, and getting a cafeteria to invest in the tools and training for no immediate return will be very difficult. Perhaps a more feasible alternative is to petition for less wasteful egg preparations, perhaps omelets, where the entire egg is used.