I've ordered a Comms Link. Well, in fact, it's an original Psion Comms Link with a free Psion Organiser CM included. Yes, that's the right way round - the cable is worth more than the Organiser now! Total cost £22, but I hope to recoup some of that by re-selling the Organiser CM. This is good news for the project, because it means all that fiddling around with machine code and SSCR now won't be necessary. The disadvantage is that the solution won't exactly be compact. Oh well.

What exactly is a Comms Link? It's a device that equips a Psion Organiser II with an RS232 plug. They were sold by Psion as a must-have Organiser accessory for the eyewatering sum of £60 in early 1988 - bear in mind the cheapest Organiser II sold for £99 at that time!

(picture of Comms Link from Centre for Computing History)
See that grey box with 'Comms Link' written on it? In there is a little chip which contains some terminal emulation/file-transfer software for the Psion, imaginatively called COMMS. If I connect the RS232 end to the Pi (via an RS232-to-TTL converter and possibly a 25-9 pin converter) and the other end to the Psion, I should be able to control a Pi terminal session using the Psion. Which is basically the aim of this project.