Thanks. Number under 50 will certainly be unreadable, as it will be in black font on black background. Fix is possible (as well as displaying number when Kindle is charging or signaling about low charge), but it's tiresome enough to being out of my current fun zone, so I hope someone else will do it (help me!
).
Just look at
/usr/share/webkit-1.0/pillow/default_status_bar.html and scripts included in this page.
In my shell script, text between SCRIPT line and last backquote before MESSAGE is plain JavaScript code executing in context of that HTML page. (Don't use line comments, as this code is glued in one big line before passing it to pillow.)