The default Kodi keymap uses the enter key in keymaps.xml so that you press enter while highlighting a movie to play, and during a movie if you press enter the OSD comes up with play/pause pre-selected so pressing enter pauses, pressing enter again unpauses.
I'd set the harmony to be a normal MCE remote and it should "just work" with a standard-ish LE image using meson-ir that understands MCE command, or worst case you can create a custom keymap on the LE side that understands all the harmony keypresses. You won't be able to teach the harmony play/pause without something that sends play/pause.