As someone who works for the AEC, part of me thinks its funny, but part downright stupid.
Tossing away a perfectly valid vote annoys me. People will later whinge and moan about the government, and quickly forget that their vote was instead INFORMAL.
You can be smart, you can be funny, and you can do stuff like that and yet still vote legally.
Heck we had
someone last night draw a gigantic cock with a Manly jersey on it on one of the LC (large) papers. Vote was still formally done, and when I was sorting it later that night I pissed myself laughing.
If people are going to do stuff like that, find out the formality of votes before hand, see what you can get away with, and do both (a valid formal vote AND something funny - if you must).
Every year at my booths we have competitions... first person to find a penis drawn on a booth wins a pack of smarties. First person to find a dick on the papers, wins a packet as well.
We have an open bowl of lollies, soft drinks and juice that I bring along, and have a bit of a party whilst unfolding/counting/sorting.