There are some hidden prefs you can use to do this. Run the following
commands in a terminal:
defaults write com.Growl.GrowlHelperApp "Custom log history 1" \
-string "$HOME/Library/Logs/Growl.log"
defaults write com.Growl.GrowlHelperApp GrowlLogType -int 1
defaults write com.Growl.GrowlHelperApp GrowlLoggingEnabled -bool YES
osascript -e 'quit app "GrowlHelperApp"'
open -ga GrowlHelperApp