As a hacky workaround you can have the JS decompressor replace img tags with a canvas and draw on that, or create a data or blob URI and apply that to the img tag (I'm not sure which would be least efficient).

I assume that is how https://uprootlabs.github.io/poly-flif/ is adding support. I might have to have a browse around that project's code when I have some free time.

I doubt it would be practical for production use, but you never know...