The OP didn't say they were using Apache Web Server.

It's bad practice to locate resources for a web app outside the web app.  I
would read up carefully on the <Context> tag of server.xml - I seem to recall
some dangers in defining a path="/" but I can't be bothered to google for it
just this moment.  I'm also not convinced that server.xml attributes will do
what you want.

It sounds like you're trying to access server-local resources outside the
Tomcat environment from a specific web app.  The easiest way I think of is to
create a context attribute in the web.xml with the fully-qualified path value
to the resource.  You can retrieve that value from the ServletContext inside
your app.

I recommend that you use forward slashes, even better, define it as a URL:
file://c:/path/to/wherever/file.ext

If you are using Apache Web Server you have better alternatives.  You can
define certain directories on your server to correspond to certain URLs in the
clients in the httpd.conf as udelram suggested.

The approaches are compatible.  You can define the URL of the referred
resource in your web.xml as the location of the resource.