CMU for CDB common user

Is there any way to authenticate a CDB common user using CMU?

When the external_name is set the alert log shows that the CN is changed to the common user's database username (with the C##) when the LDAP lookup occurs and then gets an error about the user not existing. The error makes sense because it is not looking up the DN provided in the external_name, but replaces the CN with the C## username that only exists in the database