I agree that with the existence of Live Preview, and Mobile Toolbar or cMenu on desktop (both which can have a highlight command button), this “highlighting in preview” FR is already achieved on desktop environments (or iPad if connected to external keyboard).
But for this use case…
And my own, which is to do the same on my iPhone…
the FR is incomplete, for one main reason: the on-screen keyboard, which obscures half the screen if you try to select text for highlighting.
This would be resolved if someone can create a simple mobile-focused plugin to force-hide the on-screen keyboard (perhaps until a hovering pencil button is clicked), so that I can select text in Live Preview, then press the highlight toolbar button, all without the keyboard popping up.