Forum OpenACS Q&A: Response to LDAP authentication
Read about it here
As far as LDAP in Tcl, well, this might be of some interest:
This is about adding a command to the Tcl interpreter so you can get access to the LDAP C library.
There's a Tcl shared library for LDAP on this page of LDAP software
And, just in case someone forgot about this, the ACS has an LDAP module too: see it here
(I don't know the licensing issues, etc., though this was done a while ago, so perhaps it would be GPL'd or whatever the original ACS license was?)
This message from openldap.org suggests that there is Tcl LDAP code from a guy at Neosoft (though when I looked through all the packages the LDAP packages seemed to be for Tcl 7.6, etc. and not the one this guy was talking about.
In any case, it does seem like there is some stuff out there to do LDAP with Tcl.