Sorry I can't answer your question directly, but when I tried a sample roll on my Canon iPF6100, I found that I didn't have an issue with how uncoated areas looked in terms of reflection. But the problem is that although this canvas is probably the most durable, it isn't waterproof, and if your fingers are a bit wet and you rub, ink will come off easily. If you can use the gloss optimizer over the entire image, (I think the HP has a setting for that.. no?), then this might make it more waterproof. Do you think this is the case?
I also tried the Hanhemule silver metallic and it was very similar in this regard. This one passed the crack test incredibly well so stretching it without coating is no problem, but that water issue is fairly big. The Breathing Color did well in the crack test as well, though not as well, but if that gloss optimizer would add water protection, this might be a great combo. The coating of canvas is really where the big cost factor is in terms of time, so if a printer could spit out some sort of protection over the ink right after it lays down the dots, this could be a huge thing. (of course the gloss optimizer is probably cost prohibitive if used over the entire canvas)