> Indeed, such a feature would be a welcome addition.

For situations where xcompose is not availble, why not mimic
what xcompose does, albeit with a different prefix?

    AltGr " e → ë
    AltGr / o → ø
    AltGr t e a → ☕

That last one is a local mapping I have, but you get the idea.
They're even already divided up by locale in the

   /usr/share/X11/locale/*/Compose

files, so a lot of the tedious legwork has already been done.