There are two current models that are firmware re-programmable. One is the HP10bII+; the other is the HP12c.
The HP12c is probably the most attractive to repurpose, because there is already available firmware for several Voyager calculators for several other models.
Both HP11c and HP10c are not too interesting, since why load them when you can load the HP15c - and what's more, there are versions with 3 times more registers or programming steps?
But what about the HP16c? this is a model intended for the Computer Scientist. It has not been reissued unlike the HP15c. And yet it fills a gap of fast operations in different number modes. These can be done with the HP48 family, but it is much more cumbersome than doing it with the HP16c.
So it has been done:
The problem remains with the lettering. The easy way is to take a photocopy or print out a HP16c picture at full size, and then paste it to the calculator with a transparent tape for packaging.
We have been considering creating a sillicone cover that would have a better aspect and mechanical feel: