I guess the idea is that footnote references get their literals changed to sequential numbers and a renderer is supposed to assign an implicit index to the footnote definitions?

Is that something you'd be willing to change such that the literal would be preserved on the reference? It seems odd that the two go out of sync like this and a renderer can always assign implicit indices to the references as needed. It would also make it easier to do custom superscript notes:

Hallo Welt![^A]

[^A]: "Hello World" in German.

Hallo Welt<sup><a href="#A">A</a><sup>

<small id="A">"Hello World" in German.</a>