Some updates on this as I've been testing LineageOS:
Dotterel: Plover + Arpeggiated Regular Keyboard
Dotterel (named after a bird, like "Plover" is) exists, so this same approach could be adapted possible for something of the above. It is an implementation of Plover for Android, and can take an attached keyboard's chords or can use a touch keyboard for "arpeggio mode" (where you can tap each letter and it holds it down until you hit the "space" bar to release the full chord).
I forgot about this mode but it might be an accessible example for implementing on a keyboard for chorded key entry which still might be useful: you could switch to arpeggiation mode with a key, then could hit "A" for instance and that is held down, then "B", then hit a "space" (release) key to release the "A-B" chord to whatever the user defined the chord as, which could be the word "about" or an abbreivation "Apple Berries" or whatever.
Other Existing Keyboards
A couple other Android "chorded" keyboards exist like Penti and Combo Key. I also didn't know the so-called "Hacker's Keyboard" exists, which has CTRL/ALT keys - but unfortunately only in landscape mode I think (would there be a way to get them in portrait mode?).
