Yes, while you don't need to use adb, you need to copy the keyboard files into the system folder and change permissions on the files. I used the steps outlined in a thread in the Kindle Dec forum on xda.
I use SwiftKey Tablet X and I really like it so far. It has the split landscape keyboard like thumb keyboard has, but much better prediction. However, I'm rooted and running CM7, so I can't say whether or not it works with a stock Kindle or not.