[Serusers] How to mangle SDP with result of shellscript

Hi there, I have a question concerning the following situation: One client makes a call to another client in the same (private) LAN over a SER located in the public network. It is easy to check if both clients are behind the same NAT. So one can avoid using the mediaproxy and therefore the mediastream stays in the local LAN. But there is a problem if one client uses STUN to detect the outbound IP of the NAT-Box and changes the IP-Address of his SDP body to the outbound IP-address of the NAT-Box. The original local IP-Address ist lost. This value must be recovered by SER - otherwise the RTP-stream of this local connection is relayed over the public internet. I managed getting the original IP-Address (external script which extracts the IP-Addr from the Call-ID header, if it is there...) and store it persistant in the Location Database. My question: How is it possible to mangle the SDP-body by SER with the sdp_mangle_ip() - function and a non static argument. I have to use this function with an argument given by an external script. Is it possible or does anybody know another possibility to get rid of this mess? regards, Philipp