Well, photobook (in contrib) uses CR attributes to extend users and it can be customized via the Tcl script that defines the CR type it uses. Since it uses the CMS auto-form generation stuff you don't need to change the code itself.
Missing, though, is the web interface to add or delete new attributes. The CMS has code to do this for CR types that could be woven into photobook then you'd have what you want, I think, Til ... get to it, dude! :)
