I've written 2 patches, one to make simplecache use a subdirectory of HTTP_HOST within the data directory, and one to allow code in settings.php to add a prefix, after the site prefix, to the core entitiy tables.  So whereas normally entities, users_entity and sites_entity normally get prefixed with something like 'elgg' to give elggentities, elggusers_entity and elggsites_entity, on my system, depending on vhost, that becomes something different.  So I have elggbdsites_entity on one site and elggeqsites_entity on another.  That allows me to virtualise my instance of elgg.