I don't, but according to their website's list of things they don't cover https://www.petinsurance.com/whats-not-covered/ it's not surprising heart murmurs would not be covered. I would definitely contact the breeder if you haven't already though. I know you didn't ask but I also know that treatment for issues like malocclusion are also specifically not covered either on the full list of exclusions.
I haven't had experience with Nationwide for chinchillas, but I do remember when my cat got rejected by a different insurer for a pre-existing condition. It was tough, but then a friend suggested looking into Sagicor. Surprisingly, they were super helpful and understanding. They covered my cat's treatments without any hassle.