Wayland doesn't handle custom keymaps at this point,
This is tangential to the main point of course, but I have found that Wayland will handle already-defined custom keymaps just fine, but attempting to create keymaps under Wayland via the keyboard setup does not work. So I log in to an Xorg session, set up the keymap, then I can log back in under Wayland and the mapping works fine. An example for me was setting up ALT-H to hibernate (systemctl hibernate -i). If I set up the map under Xorg, then when I am in Wayland I can ALT-H and it hibernates just fine, but trying to set up this mapping while in Wayland just doesn't work. When I go into the custom keymap setup and press ALT-H, it thinks I pressed something like ALT-SUPER-META-H (and my keyboard does not have SUPER or META keys). YMMV.