Virtual chord keyboard

I have seen Doug Engelbert's Mother of All Demos before. It's remarkable for many reasons, but I'll focus on the chord keyboard. This never took off, even if it seems like a nice idea. I would have liked to explore it more, and, of course, try it out. I don't want to buy a chord keyboard, because I already have a perfectly good keyboard on my laptop. My idea is: use four keys such as ASDF plus the spacebar as a chording keyboard for your left hand! Simple, right? Software support would be needed, and as of now, I cannot find any. A Google search turns up only unrelated musical chord stuff, and of course, hardware chorded keyboards.

Software support

We need software to translate the typed chords into characters. There are several possible software support options.

Dedicated text editor: this is the best option for a proof of concept. It's quick to implement, and it could prove the point. Of course, you could only chord-type inside it.
Keyboard layout/dedicated device driver: this is the best long-term option. It enables you to use the chorded keyboard in any context, any application. Except BIOS settings, but that's hardly an issue. However, its implementation is more difficult than a quick and dirty text editor.

Chords

How should chords be mapped to characters? And not only characters, but also numbers, punctuation, and most importantly frequently used control keys like arrows, the spacebar, backspace and enter. Providing access to these keys is very important if we want to avoid moving our left hand.

I am currently looking into chorded keyboard manuals and documentation to find best practices in this area. I would love to keep it simple and only use 4 keys + the spacebar (for the thumb) but a hugely popular layout is the 8 key Braille for blind persons. It's proven and tested, so why not? But it's hard to use with a single hand (we need to keep the right one on the mouse, remember?). The blind use it with two hands, and the keys are arranged in two outside facing semicircles. Not at all two horizontal rows, like QWER and ASDF on a PC keyboard.

Proof of Concept

As I'm only interested in trying out a chord keyboard, I'm trying to find a suitable software. But it seems there aren't any. I'll try to implement my own, as time permits. Python is my favourite language for prototyping, btw. It'll be the first time I'll use it's GUI libraries. It's already looking like a good project!

Update:
I have found Key-To-Byte an old little app that lets you emulate and try out an 8-key two handed Braille keyboard. The conclusion is: using a chorded keyboard is hard! It would take a long time to learn the keys, it would possibly make you forget touch-typing, which is a quickly forgettable skill anyway. And it's very error prone, even if mildly skilled, apparently. So it's just not worth it.