I've got a tube full of opto isolators (somewhere!) so I'll probably have a go someday if they turn up in the never-ending story of tidying my flat up and organising my electronics "collection" (read: Hoard). I mentioned the serial idea mainly because it seemed like something that could be achieved in an hour or two without too much soldering, and 4 bytes a second is still 10 times faster than I can accurately type!!
As for the Arduino, I suppose if I wanted to keep it retro I do have a literally dozens of 6502/6809/z80 microprocesors and support chips, a load of EPROMS and static RAM chips so I could make something retro custom for the job! That said, given the time its taken to make the MK14 I'd be looking at a long project
No, I like Sirius' idea better from both a technical and aesthetic point of view, and using a RPi seems like a good plan because then I can have the whole development environment and the uploader hardware in one place and just SSH into it from whatever computer I'm working at! Three 74LS138 3-to-8 decoders, a scattering of resistors and the Pi and the jobs done!