You can try "libuser", it's available in CentOS 5 (yum install libuser) and apparently has support for LDAP.
"libuser" is an attempt to generalize the useradd/mod/del, groupadd/mod/del commands to work with generic backends. The implementation includes a module to work with an LDAP backend, I just don't know how functional/stable it is...
That is a very interesting information! Thank you!