However, the idea to run OpenVPN on 53/UDP is something quite astonishing for me. It really does not attract unwanted traffic for you? :o
Don't think it'd attract anything. Those scanning the Internet for DNS servers won't get any response. It will make OpenVPN log spam "Authenticate/Decrypt packet error: packet HMAC authentication failed" when a DNS request is sent to its port, but it doesn't reply. Just looks like an IP with nothing listening to DNS scanners.