I just thought of a really dirty hack to get the percentage
As I said, the Kindle checks the minivolts of the battery every once in a while, as a percentage, and updates the battery bar. So here's two cheap options..
1) A daemon/process thread that runs in the background can occasionally check the battery level and echo it into a file like "/mnt/us/documents/batterylevel.txt" so that you can check it while you're reading. That would be kindof cool because it could also echo other system information (uptime, mem) and even give you an estimate of how many minutes (or weeks) you have left based on your usage level
and 2) (the dirtiest of all)...
The name of your kindle is saved in the fs as plain text and it displays on the upper-left on your screen (mine says "Tony's Kindle". A daemon, like the one above, could occasionally change the name of the kindle to the battery level so it displays in lue of you Kindle's actual name.
I think I'm going to start working on that :P