if it's only manual deletion you are after, you might be able to hack a way around.
If you create a subpatch (with GOP enabled) and a system that receives a call, and performs an action upon call
GOP, so that you can create it to become exactly the same size of the canvas and hide that behind the canvas, you can make a call to [r canvas-check] and according to the number of bangs printed, you know how many canvases are still hanging around. You know this, because to manually delete a canvas, you'd have to highlight the canvas with the mouse (which would also select (and eventually delete) your subpatch that you hid behind it), when you get rid of the canvas, you also delete your system that checks upon its own presence!
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