(idea already floated)
for multiple keys per row, use morse code
L, M and R from twiddler standards
If you want to represent presses with dashes,
G, K and W (and maybe O) from MORSE (preferred, no lowercase needed)
I wonder what fraction of users would ever be able to use "K" ,,, maybe form a "V" with the tip of the index finger pressing L and the edge of the palm just where the finger starts pressing R
if you want dots to represent key presses
D, r(lowercase) and U from MORSE code.