Here is one method for including an iFrame Lightbox inside a clickable inline text element …
The Discourse Footnotes Plugin accepts HTML as well as Markdown inside popup footnotes.
So for example this code …
… generates this clickable popup footnote containing a mini iFrame with the Expand content button on the top right corner …
To make this work I have (in site settings) enabled both enable markdown footnotes and display footnotes inline…
Then I added that site’s domain name (microchic.com) to the allowed iframes list in site settings …
… and also in the list of iframe origin domains in the iFrame Lightbox Plugin settings …
You can try it out live here …