I haven't found a way to have LaTeX markup in YAML headers to render properly. It may be possible by editing the source code for the pandoc filters, but it would be far easier to get a table of Unicode sequences and do it like this
I don't think you need to be knowledgeable. I think @technocrat was referring to the fact that you directly provide ï fro you i-umlaut without special syntax. It would then be correctly formatted for PDF or HTML output.
Otherwise, you should be able to use LaTeX syntex but that would only work for LaTeX output, and you need to tell Quarto (and thus pandoc) you are providing raw LaTeX.