asterisk users - Aug 2009 - exchanging CDR data between Asterisk servers

Hi!

I have the following setup:

PSTN<------>Asterisk<-----SIP-->Asterisk
              GW/LCR \ \          ...
                      \ \         ...
                       \ --SIP-->Asterisk
                        \         ...
                         ------->Asterisk


The GW-Asterisk just does the gatewaying stuff and writes the CDRs for
the billing system. The other Asterisk servers handle all the services
(IVR, REGISTER, ...)

Some scenarios require to write some additional data to the CDRs. For
outgoing calls this is not a problem (I signal the extra data in a SIP
header and set a CDR() variable in the GW asterisk).

My problem are incoming calls. The "extra data" is only known to the
service Asterisk, but the CDR is written by the GW Asterisk. Does
anybody know a method how to signal the "extra data" during the call
from the server Asterisk back to the GW Asterisk und put it into a CDR()
variable?

Regards
Klaus