xev said "pause" is 127, so I used xmodmap to set 127 to Menu:
xmodmap -e "keycode 127 = Menu"
xev than said the "pause" key is "Menu." But it didn't raise the KDE
menu the way the Window key does, even though xev also says the "Window"
key is "Menu"
I also put
xmodmap -e "keycode 127 = Menu"
into .xsession, made it executable, linked it to .Xsession, and put
keycode 127 = Menu
into ~/.Xmodmap
and when I rebooted, xev said "Pause" is "Menu" but it doesn't start the
KDE menu.
Maybe KDE subverts X11 settings.