According to
http://www.openldap.org/faq/data/cache/532.html, idassert-authzFrom is not needed in this case. Here is the example:
To allow (dumb) clients that do not perform bind to access servers that require bind (and some ssf) by asserting some static identity (the dn:<dn>, or even the anonymous mode, to implement the "sandbox" user described above) without any idassert-authzFrom rule in place:
database ldap
suffix "dc=example,dc=com"
uri "ldap://ldap.example.com"
idassert-bind bindmethod=simple
binddn="cn=Proxy,dc=example,dc=com"
credentials=proxy
authzID="dn:cn=Sandbox,dc=example,dc=com"
If no authzID is given, and mode is set to none (for instance because the remote server does not support the proxyAuthz control), the clients will be authorized as "cn=Proxy,dc=example,dc=com" even if they actually connected anonymously to the proxy. Beware that this may be a significant security breach, if that identity is granted anything but anonymous read privileges.