Hi Simon, your patch does not port all add-on functionality and since there is nothing about that in the description of the task, here is what currently missing:

1. Replacement for scene_update_post handler was not implemented, which means modifier stack of the canvas object would not be cleaned up if user removes the brush object.
2. Draw Poly Brush tool has not been ported.

Sorry I've updated the description.
The commit is more meant to be a starting point.
Since the Grease Pencil changed quite drastically, I think a complete rewrite of that part is required.
I din't know how to update the scene handler since it got removed. I found the "depsgraph_update_post", but not yet found a solution to detect wich part of the graph changed. Could need support here.