Was first looking at how to configure using xkbcomp and setxkbmap, but just went with .Xmodmap for simplicity. The Arch Linux wiki page on Xmodmap states that it's not really the best way to go, but no way tends to be.
xmodmap settings are reset by setxkbmap, which not only alters the alphanumeric keys to the values given in the map, but also resets all other keys to the startup default.
.Xmodmap configuration binds Caps Lock as the modifier key to trigger the Norwegian letters on a US keyboard layout.
! ~/.Xmodmap clear lock !Maps Caps-Lock as Level3 Shift keycode 66 = Mode_switch ISO_Level3_Shift !Norwegian alpha chars ÆØÅ keycode 47 = semicolon colon oslash Oslash keycode 48 = apostrophe quotedbl ae AE keycode 34 = bracketleft braceleft aring Aring
Caps-Lock + ' = æ
Caps-Lock + ; = ø
Caps-Lock + [ = å
Caps-Lock + Shift + ' = Æ
Caps-Lock + Shift + ; = Ø
Caps-Lock + Shift + [ = Å