Yes, I did only use Lua for prototyping right now. Your characterization is quite right: it's lightweight. I would not say "fast", though, but that's a relative term anyway. It's easy to interface from C code (I would even go as far as saying it's fun to do so).
I think you can get a quick impression of it by looking at the "reader.lua" file.
As for the UI, I always found Qt to be a bit sluggish. So I was even thinking of implementing my own UI. I'm not too sure about that, however, since it would definitely be more work than just relying on Qt standard dialogues. Qt/qvfb would also be indeed a great help.
Thanks for the tipp regarding the muPdf problem in newer revisions. I'll have a look at it.
Edit/Addendum: Hm, maybe integrating it into CoolReader would be an option. I'll have a look at it. However, there doesn't seem to be an up to date source code dump available right now. It's based on Qt.
Last edited by hawhill; 11-15-2011 at 10:18 AM.