How nice to see threads like this.
Can’t we add a button to the composer and popup a jQuery canvas editor, just by using client side code?
Something like Sketch JS, albeit with the download button saving the contents into the composer, as opposed to downloading.
More than happy to help out with a client side only demo if that’s of any interest. Which could then be hardened into a plugin if all went well.
Edit: Although over the top wrt the requirements stated here - tools like this also have save as PNG options. Opening up all sorts of collaborative efforts, in amongst other things, wiki topics.