> My first thought was to embed anchors into the external map SVG: […]

> This approach is problematic. Those anchors are only active when SVG is inline and don’t work with an <img> element.

No, this approach is fine: you just need to use <object data=…> instead of <img src=…>. <object> is the correct choice for embedding interactive SVG.

But you probably do want the SVG to be inline anyway, because it will load faster and is in this case actively the content of the page; which is the technical decision he ended up at, though perhaps for partly the wrong reason.

(There’s also <iframe>.)