I found the previous discussion I was thinking of. As I understood it, the major objection then was that in-line strings constitute their own subproblem that makes the situation more complicated. (And I agree with that.)
This feature would be most useful (and be most predictable in its effect) when disallowing in-line strings entirely – that was the use case I was imagining when I said it seems like it’s only a stone’s throw away from current state. Maybe the hypothetical compiler option could require having set $ZCODE_MAX_INLINE_STRING to zero? Or could force it to zero with a warning?