I think this is a question you'll have to address directly to the DreamHost pre-sales support team at the following url:
Include the details you've just given me.
BTW, if this is something they do allow, all they really need to do is to enable the wildcard subdomain DNS for you. At that point, you should be able to code the mod rewrite part in your own .htaccess file. What happens is that all subdomains will be directed to the same web directory and you'll have to code up the handling of the multiple subdomains yourself.
How much control are you going to give your users over their page? I'm just concerned to make sure they don't take advantage of you and either post illegal content, jeopardizing your account, or inject scripts that cause the server performance problems, also jeopardizing your account.
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