I'd also spend some time really considering the language you want to use with your customers. If you say anything along the lines of guarantee, you'll have to back that up. While you could choose to do that, realize that none of your suppliers will assist you with this. You aren't going to be able to "exchange" canvas or ink that faded early - the manufacturers specifically disclaim anything along these lines. I typically explain the testing that is done, but also throw in there that only time will tell for sure, and I've had no issues going that route. That being said, if there's an occasional issue I'd just replace it for customer satisfaction. However, if longevity testing wound up being way off on some of this stuff, like a new canvas or ink, it could be disastrous to a printing company to have to replace nearly everything under a "guarantee".
For your original question, if your partner still budge, make two identical prints. Only coat one. Do Bill's Windex testing, also bang the corners on the wall a few times, rub your fingernail across it, even spray some water on it. Like others I haven't found any canvas (outside of solvent ink) that backs up any claim of not being needed to be coated. One manufacturer started making that claim, and the others followed suit as marketing...