IIS7, Classic ASP and Output Caching Question?

Hi,

I have a question in relation to IIS7 and the new output caching feature.
We would like to use this feature to speed up our classic ASP application.
Currently we do not pass querystrings between webpages because we use search
enginge friendly pages
i.e.
http://www.mydomainname.com/article/...e-3434324.html
This would effectively bounce to a 404 page which is configured as an asp
page to get the response and redirect to the correct asp page using
server.execute
Parameters like the pageid, language, currency etc. are passed to the
corresponding page using session variables.

According to documentation I have read you can use Headers or Querystring to
determine the caching of a page in IIS7.
Because we don't use Querystrings then I wondered if this scenario is
possible using Headers.

a) Create a new module for IIS and place it at the top of the module list to
get the incoming request and tag a new request header to the request (called
say MyCache) and use the requested url as the value.
b) configure the output cache to use VaryByHeader in user mode and look for
MyCache

Would this type of thing be possible?
If so if there is no MyCache header does IIS just ignore it?
If not any idea of how I could achieve something without rewriting the
entire application?

Thanks,

S.