As it is there is really no way to search signatures other than using phpMyAdmin.
Having a simple search setting for "has signature" would be a start.
Ideally, would have a search signature containing xyz (e.g. so you can search for signatures containing IMG or URL tags).
Would be great to have a wildcard search option in User Search so you can search for all users with name containing xyz, email containing xyz, etc. not just exact match, which if you are searching you probably don't know if the first place which is why you are searching for it :).
What does this add-on do?
This add-on adds the ability to search for users to the main search.
It also allows for filtering by location, user group and active user upgrade.
This add-on also supports searching by Custom User Fields when installed in conjunction with Custom Fields.