I did this 5-6 years ago. Again...based on Batman the Animated Series. Few people noticed Harley's 'backwards' mallet. The way one would (usually) hold a murdering, over sized hammer, would have the thing facing the other way. I was liking everything else about the poses but the space between Joker and Harley needed to be filled and I didn't want to take attention away from the Joker's knife...so I just put the hammer barrel on the other end...she's just going to use it to flatten Robin in a unique way.
I initially did pieces like this because of my love of the series, which produced lots and lots of requests. I stopped doing them because I didn't want to gain a reputation based on the theme and style from someone else. Perhaps in the future I will revisit the theme with my watercolors and sectional style.