For the record, I’ve basically got this working. It’s not exactly a BSS topic, although it can be applied to BSS. I used a BSS Custom Code component with an <object> element, so I wouldn’t need to edit the code whenever the image changes. Styling is still a work in progress. The code is:
<div style="max-width: 800px">
<object type="image/svg+xml" data="influence.svg" width="100%" height="100%">
Your browser does not support SVG.
</object>
</div>
It would be nice if BSS allowed custom Options for Custom Code elements. That way, the SVG filename could be selected from the BSS UI rather than being hard-coded in the HTML. This would make it more feasible to publish Custom Code elements online for other users.
Links (<a> elements) in the SVG file need to specify target="_top"; otherwise the new content will only replace the SVG image rather than the whole page.
Getting LibreOffice Draw to produce correct SVG links is also a problem, but that’s definitely off-topic.