Hi @mbruijnpff 
That's right, the video you linked is indeed a Retool application!
We had to get pretty creative with the existing bounding box component, using a bit of JS and CSS magic to make everything work just right. But the goal you're describing should be feasible!
If you're interested in our lessons learned using the bounding box component, you can read more about it in this post: Digitising comic books with bounding boxes.
Good luck!