I can have another go at it on another PC if I get a chance, but it's not simple to add. Previous attempts were requiring too much code to tweak.
Overall C2's webgl renderer is made to work well with the screen and not arbitrary rectangles like this plugin does. Ideally I'd just call some functions from the renderer but as it turns out instead I needed to copy and heavily modify most of those functions. Pasting objects with effects is one such example and there are differences with my code that can make the pasted image not look the same. There's probably a simpler solution I haven't come up with yet but it'll probably be very hacky.
That is probably one good reason why such a plugin like this would have benifited from being made officially. They could then be able to tweak their renderer to make such a plugin more streamlined.