[sxv]

I did some back of the envelope calculations for fun. A 5-key setup would allow one-hit triggers of:

abcdefghijklmnopqrstuvwxyz and space, period, apostrophe, enter.

Then add a "shift" key and you add one-hit triggers of A-Z, backspace, question mark, tab, esc.

Then you could get 60 additional output values with double-tap shift and hold + key (30) and double-tap shift and release + key (30).

[pcwalton]

That's essentially 5-bit Baudot code: https://en.wikipedia.org/wiki/Baudot_code