Another possibility is for the host to be able to make demands of the UMLs. It could tell a UML to release a certain number of pages in a certain amount of time. The threat behind the request would be that, if the UML doesn't comply, the host will choose pages from the UML and swap them out. Since the UML has better information about what it doesn't need, it would presumably be in its own interest to comply.
An elaboration of this would allow the host to specify whether it needs clean pages or whether dirty pages are OK. The host may need clean pages if it currently has a memory shortage, and there either isn't time or IO bandwidth to swap out pages. It could specify that dirty pages are OK if there isn't an emergency, and there is time and bandwidth to swap out data.