What you are looking for is a DNS service that whose nameservers you can use, and customize. Since you have registered your domian with DreamHost, you would then put that services nameservers into your whois information at DreamHost, and do your cname customization to point it to smugmug on that service (if, in fact “power” users at smugmug can even do that - I don’t know).
One such DNS service is http://www.tzo.com/ , though I do not “recommend” them (I have never used them). Hopefully some users here who do use such services could share their information.
Note that some other registrars provide more flexibioity in customizing DNS than DreamHost does for “non-hosting” or “registration only” accounts. If you do not plan to host at DreamHost and want to keep you actual site at smugmug, maybe such a registrar would better suit your needs.