I kind of see where you are coming from and I agree that it does not hurt to ask but at the end of the day, I think it may be a technical issue.
I was invited to paint on leather luggage tags last year at a Moynat event. And it was actually quite difficult. We used an acrylic-based paint. There had to be at least 3-4 layers of paint and they needed to be painted in a way that the paint had to saturate in between all the natural texture of the leather. I'm sure canvas has a different texture and is probably easier to paint, but it still requires layers of paint. At the end, it gets a very durable clear "top coat". I've spoken to my local Moynat painter a few times about the process. He said once you put on the top coat, it basically will stay there forever. It should be waterproof, crease resistant, and scratch-resistant.
To paint on top of it, I have a feeling, would mean to paint on top of that top coat which may meant that the paint may not adhere as well as it does to the canvas. If the top coat needs to be removed, then it needs to have non-water-based solvent to remove that top coat, and then a water-based solvent to remove the original paint.
But using a non-water-based solvent may damaged the coated canvas itself, since there is a coating on the canvas as well.
I'm not sure if this has been attempted before and they realized the difficulty of the work or that it does not work at all. Or, based on the paints and top coats they use, they are aware of the limitations of what can and cannot be done.
When I watch the LV trunk painters on IG, when they make a mistake or they're not happy with their work, they basically erase the mistake (or erase everything), and start all over. But that is prior to putting on that top coat that sets and protects the paint.
Just some thoughts though. I'm not a professional painter by any means.