Rewriting subdomains

Hi All

My hosting provider doesn't have rewrite logging turned on, so I'm
flying blind on this problem.

I am trying to implement a structure where I have:
A domain host1.com on a shared hosting provider.
A virtual domain host2.com pointing to the same IP address as host1.com.

A directory structure

host1.com
app
www
docroot
demo
docroot
test
docroot
dev
docroot

I want to map http://test.host2.com => host1.com/app/test/docroot

(similarly for www, demo, and dev).
There is more path to the absolute root above host1.com

I have been trying variations of:

RewriteEngine On
Options FollowSymLinks ExecCGI -Multiviews

RewriteBase host1.com

RewriteCond %{HTTP_HOST} ^(.*)\.app\.com$
RewriteRule ^.*$ /host2.com/app/%1/docroot/index.html


Is there a stock solution to this problem?

Thanks.

Jim Rootham