In general, when something is draggable and droppable it's easy to understand it.
Per example when a new user user came on your website or app... you can show a small box to show to users that your canvas are draggable, just to be sure that if people want to use Drag & Drop they don't have to look if they can or not.
Custom pointer seems to me a good idea, there is no such things on Google Drive and yet open and closed hand are pretty obvious.
Also, you need to select an other way to move your canvas than Drag & Drop, to let choice to users. Eg: For upload a file you can Drag & Drop or Browse your files, in a different way you have to provide different kind of usability.
To get back to your main question, in my opinion, custom pointers are efficient but I think you will need an other hint to show Draggable Canvas. But as always, it depends of the kind of people you are targeting. Who will be your users ?