If you were to run a .jar file through a file converter, you could turn it into a .zip file. then its as easy as finding the file named 'steve' in the entities and replacing that with the skin of your choice... the catch is that you have to rename your chosen file 'steve'
then you co back to the file converter and reconvert the .zip file into a .jar file... the next time you get on, you should be in your chosen skin.
I MAKE NO GUARANTEES!!!! THIS IS ONLY A THEORY AS TO HOW TO DO IT!!!!
If someone would like to try, then the file converter I recommend is
www.convertfiles.com. I have been using their services for years, and they have never let me down.