It's not that simple. Advertising a supportedExtension in the rootDSE implies that the
entire server supports them. slapd allows multiple backends to operate at once, and what
you're talking about would only be valid for a single specific back-ldap instance.
The standard format for extended ops doesn't include a DN, and slapd uses the DN to
determine which backend should process an incoming op. So there's no generic way
for slapd to correctly forward an incoming exop to the correct backend, or the proxy backend.
Every op must be handled explicitly and must be at least parsed in order for slapd to
be able to route it.