Solved
You can't, if using Direct Guest Join.
DGJ for Zoom Rooms uses a portable Chromium browser under the hood that takes a single camera (which you can't even choose/switch) and is not a Zoom meeting, so you won't have Zoom-specific functions.
If you use Pexip for Zoom Rooms, you'll be able to add multi-cameras, since an intermediary Zoom Meeting is stood up before connecting everything over to Teams via CVI. However, this solution has plenty of drawbacks as well and your layout, while multi-cam, will be condensed into a single MS Teams participant window.
Sorry to be the bearer of bad news (as of the time of this post), but them's the breaks.
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