Aha! An achievable!
That's to say, I may have been underthinking this. If there's something that I believe I am teaching, then presumably there is some idea of progress within a given class, which ideally one can write down as a process - the steps from here to there that are this "progress" - a lesson plan. If one is aware of their lesson targets, what they are "trying to teach", then perhaps developing a particular lesson as a demo isn't so hard.
If that's a bit abstract, what I meant was I had an A Ha moment reading the description of your demo lesson, Late. Especially the sentence "[Grammar] focus is on...." A Ha, I thought, lesson focus! What's the intended achievement! And for a demo lesson, what targets do I want to show off!
I still don't know, of course. I think for the non-language classes I do, the target is "the 4 skills", so perhaps some demo around introducing and using those. Bingo!