You read my mind. I was trying to do the exact same thing, using custom_post.conf as well as just custom.conf, with or without the (+) and it just doesn’t work for the pjsip includes. I can do this all day long with sip_custom_post.conf so either there is a different method for pjsip files, or this is not currently supported.
Edit 3 years later- this is now possible, you put the type in parentheses: Override pjsip.identify.conf - #12 by jcolp