"/chatlog" is overwriting, not appending

I use "/chatlog" to record chat, but I've noticed that when I log a different character and use "/chatlog" again, the previous log is overwritten (unless I take the time to move the log file out of World Of Warcraft/Logs, first, which is a hassle if I'm relogging several alts to move inventory).

Is there an option I can set, somewhere, that will make WoW append to the existing WoWChatLog.txt rather than overwrite it?

Ultimately what i want to do is record all chat, all the time, and have it saved automatically to file so that I can filter it later. I play WoW on a Macintosh, System 10.5 (Leopard).

What I definitely don't want is to lose chatlogs because I relogged a different character and started logging chat again.

Does anyone have any suggestions or solutions?