overriding document root dynamically?

[Jonathan Swartz]

I have an application where I want the effective DocumentRoot to change 
based on dynamic properties in the request.

In particular, we are creating a number of domain aliases, pointing to the 
same IP address, so that each user can view their own version of the web 
site. e.g.

   joe.dev.mysite.com would have doc root /home/joe/www
  dave.dev.mysite.com would have doc root /home/dave/www

etc. I could do this with a set of virtual servers, but then I have to 
change the httpd.conf and restart the server every time a user is
added, which is undesirable.

Here's what I wanted to work:

    sub trans_handler
    {
        my ($r) = @_;
        my ($user) = ($r->header_in('Host') =~ /^[^\.]+/);
        $r->document_root("/home/$user/www");
        return DECLINED;
    }
    
    PerlTransHandler trans_handler

but I got

    [error] Usage: Apache::document_root(r) at handler.pl line 41

so document_root ain't writable.

Any other suggestions?  I'm loathe to recreate the entire default Apache 
directory handler in my trans_handler (looking for index.html, etc.)

Jon