For the purpose of http caching with varnish, it should be possible to specify, at the siteaccess/group level, custom http header(s) to send.
This would make it possible, to selectively purge all http cache for a given siteaccess(/group), regardless of any host/uri configuration.

It seems the only workaround to this feature is to use multiple varnish instances, but this may not always be desirable/possible.