That would require the application to support it. Like, "got anything you can dealloc?" "Oh shoot, now you mention it..." What exists already in the Windows VM

That would require the application to support it. Like, "got anything you can dealloc?" "Oh shoot, now you mention it..." What exists already in the Windows VMM, and I assume other OSes VMMs, is a distinction between allocated and committed. That's likely to be the best that's realistically achievable. The win VMM also has a last-accessed strategy for paging to disk, and de-duplication of shared pages.