If you were wondering what happened to this project: yes, it was on hold for a while for various reasons, but now it's back! 

The first version of the calculator is still alive, but predictably the stickers have worn out quite quickly, so for revision 2 I decided to try a different approach: the front panel and keys both made of PCB material, with labels printed as a silkscreen. And I think it worked out quite well as a proof of concept, see the photo here.

This version has improvements in both the firmware and (especially) in hardware and enclosure design. I consider it a transitional revision and already see some further improvements to be made for rev. 3. However, I'm going to document this design here anyway, including some pitfalls that I came across.