Switching between one and two columns — paradigms/solutions?

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I am typesetting PDF of a book I have, and I have come across a layout I'm not sure how to achieve. It's a page that starts with one column, but then shows an example figure, which is typeset in two columns (see image below). I thought of some ideas, e.g. Define a new master with the two column frames at 100% height, with a single column frame on top that expands as it fills with content (like the frame for footnotes does, but on top). I think that would work, but I foresee problems if the example figure content spills onto the next page.

I also want to ask what SILE's paradigm for typesetting this kind of thing would/should/could be. In LaTeX, I guess we have minipages or commands that switch between column layouts (I guess; I haven't typeset this sort of thing in LaTeX before). If I were typesetting this content in HTML, it would be quite straightforward — wrap the figure in a div and use CSS columns, flex, or even grid. What solutions are currently available in SILE? Are there existing paradigms from programs like InDesign? If I could have the flexibility of something like the HTML/CSS method, I would be a happy camper indeed!