ct_sync, kernel 2.6.10, NAT and masquerade
Following up some thorough testing and debugging, I finally got both (SNAT, DNAT) and MASQUERADe to work with ct_sync on a 2.6.10 kernel.
Apart from forgetting to disable TCP window tracking, there were some subtle mistakes in #ifdef/endif of the code that actually prevented whole sections from being built ;)
Debugging the problem however has forced me to update the ct_sync ethereal plugin (screenshot) to parse almost every bit within the ct_sync protocol.