with very bleeding edge code (you need to build master yourself, not in any release)
basic idea:
after the call is established
use the use rtpengine_subscribe_request get the SDP needed to send to the siprec server
to get the sdp you need to send to the sip rec server, the last parameter of this you specify an xavp where you determine which stream labels are associated with which endpoint. This can be used to build the xml metadata for the siprec server.
send the request using t_uac_send
use the local event handler on the outbound invite to save the original dialog information into the new transaction, and arm a response handler to get the response from the sip rec server
on the local ack, save set the dialog to be for the orignal call so you can save all the routset information (route, contact, rui, csec) as variables in the original call dialog. This can be used so you can correctly build an in-dialog bye to the sip-rec server at the end of the call.
I hope this helps!
Regards,
Torrey