Yay! I have it working on my machine :-)
In order to get a clean grunt run, I also copied toolbox.xml into the blockly folder in build and changed ui_blockly.js to load it from there. I then removed the blockly folder from src.
I don't know much about blockly so I wasn't able to do much. But clearly the basic functionality works and it looks like great fun, Ruben.
I think the subplugin idea is the way to go, but I haven't had time to look into whether that can be made to work with question types and what I have to do to support it. Let us know about any further developments. I suggest you email me directly with minor tweaks, bug fixes etc but post back here if you have actual developments suitable for others to use.
Thanks again for an interesting and fun development.