No, I don’t mean the www sub-domain. While www is in every sense a sub-domain; when domains are used as primarily web-only devices, webhosts and end-users generally treat the fully qualified domain.tld and www.domain.tld as being the same entity.
At Dreamhost every domain and sub-domain you create in Panel is an entity in it’s own right at the point of creation. You can create x sub-domains and have them controlled by y different users. In your particular case this means you can have your primary domain redirected to another Host and still host multiple sub-domains right here at Dreamhost under your current, or different, user(s).
Just set up a sub.domain.tld under a user and set it as Fully Hosted.
Utilising a sub.dreamhosters.com sub-domain for your development purposes can help make migrating tasks a lot easier to follow when the time comes to move sites from Host to Host.