It seems that zimbra doesn't use memberOf attribute anywhere, so would it
be acceptable to use it for own purpose withour breaking anything?

eg. I would like to organize my domains with tags: gold, silver, bronze and do an ldapquery from another server to see which domain belongs to which group.

Or is there some documentation about ldap-schema if there are some attributes what I could use?

or if there is any other way to do this, please tell