I was reading some old posts to this blog when I came across a good idea that I'd since forgotten: separate the external I/O driver Vdd from the internal logic Vdd. This would allow the external I/Os to be driven to 3.3V instead of 5V, easing the interface to modern electronic components such as FPGAs. I haven't tested this idea on a breadboard yet, but it seems like it ought to work. I'm writing this posting so I don't forget the idea again.
And what about the EP-102? I got it working, mostly. There's a weird hiccup that causes additional lines to be "printed" to the serial port, but I suspect that's a glitch in the test module and not the printer emulation.
I also looked at some LCD and OLED character modules. I'm torn between 20x1, 20x2, and 16x2 modules. The 20x1 would be most readable, but might not fit in the space available. A second line would allow me to display other info like the OVR indicator, along with the characters that should be in columns 17 and 18 in the case of a 16x2. I'm also somewhat constrained by the blue-colored window in the calculator's case.