Use standard Unicode code points that you can link custom glyphs. You could, for example, use the A-Z and a-z codes and change them with your own words. After that, you'd keep typing with your font on a standard English keyboard.
There's no doubt that this is true for other writing systems. You need to press alpha through omega (-) on a Greek keyboard, while on a Russian keyboard, you need to press a-.
Unicode's "Private Use Area" codepoints can be used (PUA). These places have been set aside for people to do as they please. In some ways, it's a good match. However, until you make a separate "keyboard" driver for it, you won't be able to use it to write.
