I've actually done that with another project I worked on. The one issue I ran into is that an email address can be changed, so it couldn't be used as the actual userid (changing the user id on SharePoint requires an administrator to migrate the user). I
ended up just generating a guid for the user id, and using the email address to look up the user id when the user logged in. The one problem with this though is that there are several places in SharePoint where the user ID is show instead of the users
full name - and because of the guid it's unrecognizable at a glance who the user is.
Raise it as a feature request in the issue tracker and i'll consider it for a future release.