I built one of these! With a shifter on the palm, and a separate control toggle, you'd get pretty much the full ASCII set.

Even though I'm right handed, it worked best on my left hand. This is because of how surprisingly nice it was to use my dominant hand to do something (use a mouse, eat, etc), while still typing with my left.

Alas I assume the reason this never caught on was due to the learning curve. People will never leave qwerty. Also it was pretty slow. When I built this back in 2005, I was thinking for PDAs and early cell phones and was only competing against T9 and early palm keyboards. After a few months of practice, I never topped about 45 wpm.