Rtsp over http, Custom http header field

Hi,

Iam looking into adding a custom header field to http requests made by rtspsrc when using rtsp over http.
There is a call-back "before-send" which lets applications modify messages before they are sent, unfortunately this callback is only for rtsp messages.

Does anyone have an idea on how to add the capability to add custom header fields to http requests in rtspsrc in an up-streamable manner?
Or is there already a way that I am missing?

Kind Regards
Daniel Moberg