While there are RFE's like
Bug 7235 - Auto Provision New Accounts with External LDAP
Most hosting providers make their own sign up pages, that script zmprov commands/SOAP to create accounts & assign passwords etc.
(See some of the portal management techniques of the
HSPs/VARs)
Yes bugzilla is several versions out of date from the current release, but there's tons of custom stuff put into it, so it's no small task to upgrade.