This is one of many things that used to be easier before wayland

You can get pretty far with just the xkbd definions though, although you need root to reconfigure it.

A bit ironically, the easiest way I've found to create xkbd definions is to start an X11 server, reconfigure your xmodmap+xcompose, and export the current xkbd settings. Xmodmap is much easier to edit than editing xkbd directly.