[kps]

> Xcompose only offers sequences started with the "compose" key (whatever it is mapped to)

Not correct. For example, on my usual keyboard layouts¹, I use sequences starting with <dead_A> and <dead_U> for superscripts and subscripts.

> and resulting in a single X key-symbol

Also not correct. I certainly have a few non-single-keysim entries in my .Xcompose, e.g. <Multi_key> <R> <2> ↦ ℝ².

¹ https://github.com/datatravelandexperiments/kps-keyboard-lay...

[keybored]

Indeed. In my experience you can use whatever mappings you want. Just grab a writing script that you never type in and go ham : `<Ethiopian_B> …` (Note: made up keysym).