That's not gonig to work. You don't get a "super user" that can manage files owned by another user. What you do is login using their FTP username and password; either that or wait until ACLs are supported.
Also you need to diferentiate between WebIds and users. WebIds are used to manage your DreamHost account and plans. Users are used to access system resources, such as FTP, mail, and shell. They are not the same thing.
As for as restricting what a WebId can do, sorry I don't have enough experience with granting privileges to other WebIds.
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