More information about the design and the rationale behind the LDAP Identities code can be found at the pages Repfilter Sendmail Milter and its design.
An alternative approach to storing Squirrelmail Preferences in LDAP is the ldap_prefs_backend plugin by Daniel Marczisovszky. This plugin features a different schema, will save all the preferences that are available, and has some other differences in design. You can check it out to see if it suits your needs better.