The easiest way to do what you want is just to change the “web directory” that is used to serve the sub-domain.
You can do this via the Manage Domains screen of the Dreamhost Control Panel. When you browse to the Manage Domains screen, and add a sub-domain (in your example “parker.domain.com”), you can specify what directory in your user space you want to be used to serve the domain/sub-domain from. By default, DH uses “sub-domain.domain.com” in your user space as this directory, but you can set it to whatever you want.
For example, DH would by default set the directory for the sub-domain parker.domain.com to “/home/username/parker.domain.com” , similar to the way it set the web directory for “domain.com” to “/home/username/domain.com/”.
If you wish, you can change the web directory for parker.domain.com to be /home/username/domain.com/parker/ . You can define the web directory at domain/sub-domain creation time, or you can change it later by clicking the “edit” link to the right of the domain or sub-domain name in the Manage Domains screen.
This will accomplish what you have described, and as you can see, you will not need to use re-write rules to accomplish this. While this will work, I do not think it is the best way to do this, because “parker.domain.com”, and other domains set up this way, will the inherit the .htaccess settings and permissions of the “domain.com” directory in which they are contained.
Since Dreamhost allows you to “fully host” an unlimited number of sub-domains at no additional cost, I think it is much better to simply create a “parker.domain.com” fully hosted domain/sub-domain and set it 's web directory to the default “/home/username/parker.domain.com” so that, as you later develop both domain.com and parker.domain.com, you avoid the potential of conflicting .htaccess interactions (each domain/sub-domain will have its own directory “tree” beneath your user). Either way you set it up, you will use the same urls to reference the files; the differences will be in that you have a “cleaner” directory structure that could become very important as the sites evolve and various applications need to have their own .htaccess re-write rules, permissions,and other settings.
If you set all these sub-domains up as the same user, there is no disadvantage to doing this (and if you have wildcard DNS setup, it will still work). I suspect that many think of hosting a “subdomain” as a “subdirectory” and then re-writing via .htaccess as that is the only method they had available without extra hosting costs on a previous host. On Dreamhost, there really is not a reason to do it that way, and Apache is more efficient without re-writing urls.
Keeping a “site” within it’s own directory tree is just a better way to go, and gives you a lot more flexibility in managing users and security than complicated groups and permissions schemes and url re-writing if you have the ability to do so (and on Dreamhost you do have that ability).