How to get current DocumentRoot for VirtualHost in Config Handler

[Christian Wetzig]

Hi,

i have an config handler like in
http://perl.apache.org/docs/2.0/user/config/custom.html#Creating_and_Using_Custom_Configuration_Directives

is it somehow possible to lookup the current DocumentRoot from within a
directive handler like the following

  sub MyParameter {
      my ($self, $parms, @args) = @_;
      # somehow get DocumentRoot (not ServerRoot)
  }

i've tried various approaches and looked through api-docs, but without success 
:-(

$parms->server->server_hostname gives me the current VirtualHost, but i see no
method for looking up DocumentRoot.

Thanks for any hints.

best regards,
Christian