The problem is that the IP might change. Our system generally assumes that we're authoritative for the domain. Adding a unique IP will reduce the chances that your IP will change without warning, but it could still happen.
You could add a unique IP to a 'blah.dreamhost.com' domain, setup the domain as a mirror, and then make www.example.com a CNAME to blah.dreamhost.com. However, since you can't have a CNAME at a zone's apex (i.e., example.com itself can't be a CNAME), you wouldn't be able to point just example.com there.
By the way, w/r/t adding a subdomain without adding the domain itself, there's currently no good way to do this, so you'd need to add the domain, disable all services for it, and then add the subdomain. Make sure to disable the mail and DNS services especially.