I think it would be useful to be able to search RGs by status, in addition to type. I understand that status is an attribute of the release but many RGs have releases of only a single type and this would allow you to search for RGs that have at least one official release, for example.

e.g. I would like this search to work (and return only a single result: http://musicbrainz.org/release-group/36be9276-38fb-36eb-a116-9b2fc725d2e6 )
http://musicbrainz.org/search?query=artist%3A%22nine+inch+nails%22+AND+type%3Alive+AND+status%3Aofficial&type=release_group&limit=25

the wider search has many bootlegs
http://musicbrainz.org/search?query=artist%3A%22nine+inch+nails%22+AND+type%3Alive&type=release_group

This would work similarly to the "reid" part of the index, which presumably is a set of IDs for the releases.