Make Caps Lock an additional Menu key

Use the localectl command.

localectl [--no-convert] set-x11-keymap layout [model [variant [options]]]

To make caps lock an additional menu key, the options parameter should be caps:menu

To figure out what the other parameters should be, enter this command:

setxkbmap -print -verbose 10

The command I ended up with was

localectl --no-convert set-x11-keymap gb pc105 mac caps:menu

See Keyboard configuration in Xorg for more details.