Someone figured out a clever hack to get this to work: Use mermaid with code fences
It depends on goldmark converting the fenced code block (with an unrecognized language) to a <pre class="mermaid-language"> rather than the mermaid default of class="mermaid", so it might break in the future…
But now I can use Typora to edit diagrams offline, and see the expected diagram in the blog, and that’s good!