A reverse caching proxy stores the data in a transient cache. This might not be what you want when saying "exporting most information" from GAL.
People can then use the OpenLDAP server as a Directory
source in Outlook
without needing to log in to anything.
If that's your main goal you can use a reverse proxy. If you want to use the proxycache overlay slapo-pcache is the 2nd question.
See man slapd-ldap.5 for back-ldap and look at configuration directives starting with "idassert".
Thanks for the info, I will look into that!
Chris