Creating a kext to swap the shift keys with the caps lock key or the function (fn) key

I have a problem with the shift keys, and I would like to be able to swap the shift keys with the caps lock key. I didn't find any utility that would permit me to do that; the only solution I have is to create a kext.

Why doesn't a kext already exist to do that?

I found DoubleCommand, but between all the possible swaps it can do, there isn't the one I need. Are there any limitations that don't allow to create a kext for the pu开发者_StackOverflow中文版rpose I described?

Have you tried this one?

http://pqrs.org/macosx/keyremap4macbook/

(Open source in case you're curious how it works.)