I am super glad you like it! The primary reason I haven't introduced support for iframes is that it requires much wider permissions (instead of just access to the current tab after clicking the "ARIA DevTools" icon you'd need to grant access to all data on every website you visit). I will research if things changed since I last checked, though.

I wonder if you're able to open the iframe in a new tab and then use the extension on it?

Yes, that’s possible with StoryBook. It’s one of the tiny buttons on the top right, I think.

Mhm, also works in Playroom: Preview -> Open