I have used TouchKeys on Windows, which doesn't do exactly what you ask for, but is also pretty nice: it makes Space into a modifier key, and you can configure

I have used TouchKeys on Windows, which doesn't do exactly what you ask for, but is also pretty nice: it makes Space into a modifier key, and you can configure the combinations. E.g. Space+J could be cursor down, or Space+G could be escape. Tapping Space itself just produces a space. But it does mean that you lose the key repeat functionality of the space bar. So I used Space+S to produce space, and that works well with key repeat.