Hello Koen,
I'm not aware of any, but that sounds super cool! I would love to be able to draw sketches on/in the blockly workspace (as well as in my text-based code, but that's just dreaming hehe).
One thing that comes to mind which is similar is this is some people have
modified block tooltips to allow them to show images/gifs. But that's more of a docs thing than a comment thing hehe.
Also I'll say that if you want to do this, it's probably possible, but not without forking/monkey patching blockly. The icons (mutators, comments and warnings) don't have any API. There are actually
hard-coded properties associated with them haha. But hopefully in the future that will be better =)
I'd love to hear more of your thoughts on this as well!
Best wishes,
Beka