NOTE: This is a conjecture, not a review, but it looks promising. (If you’re wondering how I figured this, I brought up the full-sized image of the keyboard and waved my typing stick in front of the screen.)
Every once in awhile, something deliberately designed for two-handed people looks as if it could be appropriated by people who aren’t. The TypeMatrix keyboard (especially the 2020, their old version) might be one of those things. It is NOT a viable one-handed keyboard because the alphabet is split, with the large middle keys making it impossible to type centered. However, it just might work for one-fingered or stick typing with Sticky Keys enabled. (By “stick” I’m thinking of one strapped to the hand, because it’s the only kind I’ve used. I don’t know what the ergonomic requirements are for a mouthstick or headstick; you’d be a better judge of that.)
For starters, the TypeMatrix looks slightly more compact than a standard keyboard — maybe a little smaller than a laptop keyboard. It also appears to use scissor switches like a laptop keyboard, so the key action might not be too bad. While the split QWERTY design would mean a few lifts of the stick, the reach would be a small one. The reach could be mitigated further by the use of word completion and shorthand expansion, setting your system to a one-handed Dvorak layout, or both.
Furthermore, the reach is not entirely a bad thing for stick typing. I say this because the center keys that separate the alphabet are quite useful — large Enter, Tab, Space, and Backspace keys. There are two Backspace keys, bordering either side of the alphabet on the inside. Likewise with the Space keys. The Shift keys are located on the outer edges of both sides, and are large. The Control, Alt, Caps and Function (for activating the numeric keypad assigned to the letters on the right-hand side, and thus Mouse Keys) are grouped in the bottom left-hand corner. I don’t know if the Function key sticks, but I imagine it would have to. The top row of function keys is pretty much your standard arrangement. My only quibble is that it would be a pain to end a sentence with a word ending in a left-hand letter — the punctuation, of course, is all on the right-hand side. However, a one-handed Dvorak layout might prove more conducive.
In short, the TypeMatrix has definite possibilities, but I can’t afford to try it. (The old version, however, is on sale at their website for $50 if you’re feeling adventurous. If someone has used the TypeMatrix as a stick keyboard, I’d like to hear how it is.)