Though this doesn't have much to do with IPMASQ, here are a few ideas. If you know of a better solution, please email the author of this HOWTO so it can be added to the HOWTO.
Idea #1: Say you want to log ALL traffic going out onto the Internet. You can setup a firewall rule to ACCEPT PORT 80 traffic with with the SYN bit set and LOG it. Now mind you, this can create VERY large log files.
Idea #2: You could run the command "ipchains -L -M" once a second and log all of those entries. You could then write a program to merge this information into one large file.