Since that one was closed and there is no possibility to pick up the conversation there:
Does anyone know whether adding Header on the callee’s channel has been added in the meantime?
Furthermore: going through a predial-hook (just like with PJSIP and INVITE) does not work with the Transfer function since there are no additional options like ‘Gosub’, right?
You can flag it, in the other category, and ask for it to reopened, so there is a possibility.
It doesn’t work, but not for that reason. Transfer is done on the channel that would need the headers applied, so wouldn’t need the equivalent of a pre-dial subroutine. The actual reason is that the code simply does not support adding headers at that stage of a call.