Apologies, I missed that this was a question for c2 plugins. There is no way to make custom interfaces in the editor with that, and they aren’t working on c2 anymore so there’s no possibility of doing that in the future.
You could make a separate program that modifies capx files, but you’d have to close the project in c2 first so that workflow isn’t ideal.
Another idea is to have an editor at runtime. You could do whatever you want there. probably pause the game, and draw your editor on a div on top of the game canvas. The only annoyance there is to get the changes back into your capx you’d need to either have it download a file that you’d need to manually reload into your project or maybe copy a text of the data and manually paste that into c2 somewhere.
Either isn’t super useful.
In c3 like I said you could create a div, add it to the body, set its position to absolute and make it cover the screen, and set its z index to be higher than 10000 or so so other editor elements aren’t clickable through it.