Currently in Odyssey password_passthrough works only for cleartext auth, PAM and LDAP.
Probably, we could make this for MD5 work too. And it's the whole purpose of SCRAM to defeat this feature :)
The other way to do so is auth_query - you provide a storage password to access auth data of the DB. This works for MD5 auth. When user wants to authenticate we just check credentials against what we see in the database.