As Milo has mentioned custom HTML is not an option on cloud instances
Unless you make your own "cloud" plugin using Atlassian Connect, https://developer.atlassian.com/static/connect/docs/latest/index.html
So, the quick answer is "no" with the configuration and setup I have to work with. (I had been hoping to have an easy way to display additional information within a wiki page as a pop-up, rather than statically having it take up space on the page.) At some point when I'm not developing application code, I'll have to look into Atlassian Connect. I suspect it's going to be one of those things that ends up on the list for "some day, when I have nothing to do and a staff to do it with" ...
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