Since I often use moodle as a front end for my webwork courses this hasn't been high on my personal list of things that need to be done. (Moodle already allows for email registration -- then the student has access to webwork through moodle).
However, to add email self-registration to WeBWorK itself would probably not be that difficult. The module needed is a subclass of WeBWorKAuthen.pm and would be placed in the WeBWorK/Authen directory.
There are currently examples that allow verification using LDAP and Cosign and modified verifications when using Proctors. Also an obsolete protocol for Moodle for an earlier bridge between Moodle and WeBWorK.
The LDAP and Cosign don't require additional input from the user, but Proctor does (see ContentGenerator/LoginProctor) -- so that could be mined for ideas for the user interface for self-registratering students.
It's a project, but not an enormous project in my view. I'm happy to answer further questions if someone is interested in undertaking it.