Current RouterOS stores IP address of resolver and uses hardcoded port 53. Changing it to store IP address and port doesn’t sound like anything big. But I guess dstnat would be enough. It’s just that as it is now, you can do it only for clients, not for router itself. If router requires resolver on alternative port for own use, or if you want alternative port and also router as resolver for clients (because of caching, or because you want to override some records), you can’t do it. It would require support for dstnat in output chain.