One way to reduce repetitive-strain injury from Emacs is to avoid pressing two keys simultaneously with the same hand. That is, avoid using modifier keys. For C-KEY, use one hand for KEY and the other for Ctrl. For M-KEY, do similarly as for C-KEY, or press and release Esc and then press KEY. And of course, take advantage of the automatic indenting and other features of Emacs that save keystrokes.

I've got it. There is a utility called xbkeys for setting accessibiltiy options for xorg.

From xbkey manual:

To switch sticky keys on or off, and optionally set or reset:
() two keys pressed at the same time stops sticky keys;
() a modifier pressed twice will be locked:

    [-]{sticky|st} [[-]twokey|[-]latchlock]...

I ran this:

$ xkbset sticky -twokey -latchlock

That command can go in .xinitrc

# turn on stickykeys. don't let two keys pressed at the same time disable it.
# don't turn on "latch lock", ie pressing a modifier key twice "locks" it on.
xkbset accessx sticky -twokey -latchlock

# don't expire these settings. (run xkbset q exp for details.)
xkbset exp 1 =accessx =sticky =twokey =latchlock