Linux

I'm pretty sure both are possible in xkb. But you'll have to learn how to get a custom xkb_keymap into your DE of choice. I only learned enough to do one mapping:

xkb_keymap {
	xkb_keycodes  { include "evdev+aliases(qwerty)" };
	xkb_types     { include "complete" };
	xkb_compat    { include "complete" };
	xkb_symbols   {
		include "pc+us+inet(evdev)"
		key  {
			type= "TWO_LEVEL",
			symbols[Group1] = [  Multi_key,                Caps_Lock ],
			actions[Group1] = [ NoAction(), LockMods(modifiers=Lock) ]
		};
		key  {[ Escape ]};
	};
	xkb_geometry  { include "pc(pc105)" };
};

This remaps Capslock to Escape, Escape to Compose, and Shift+Escape to Capslock. Not what you want, but hopefully this will give you a starting point to playing/breaking xkb.

Another benefit of doing this with xkb: it's now a separate codebase from X.org, and is used in every Wayland compositor I know of.