Open a terminal and drop this in:
touch ~/Library/Logs/Growl.log
defaults write com.Growl.GrowlHelperApp GrowlLoggingEnabled -bool YES
defaults write com.Growl.GrowlHelperApp GrowlLogType 1
defaults write com.Growl.GrowlHelperApp "Custom log history 1" $HOME/Library/Logs/Growl.log
Then you’ll find the log file at $HOME/Library/Logs/Growl.log and you can parse it or do whatever, like display messages on your desktop with GeekTools or something.
:A