I was able to figure out a way to set up the mailing list prior to moving DNS to DreamHost. I'd already created the list via the DreamHost control panel, but couldn't access the admin pages for the new list.
a) Find the IP of my discussion list server. Since I have discussion lists on DreamHost already, I did this by pinging lists.olddomain.com.
b) Add lists.newdomain.com (DNS for newdomain.com still points elsewhere) to my hosts file, and point it to the same IP as my active discussion lists.
c) Go to http://lists.newdomain.com/admin.cgi in my browser, and the admin interface for my new domain is there.
I completed setup for the new lists. If I didn't make any mistakes, they will be up and running as soon as I move DNS.