How do I provide basic http authentication for static Tomcat webapps without changing tomcat-users.xml?

I have access to the Tomcat manager and can upload war-files. One of these wars is a static web project (zipped html + media files, renamed to *.war). I want add a Web-INF/web.xml file to this war to protect the content with basic http auth.

I know how to do this by adding global users and assigning roles in the tomcat-users.xml, but I want to have all usernames and passwords defined in my war-file.

1. Can this be done without touching the Tomcat's tomcat-users.xml?
2. And if yes, how do I specify this in my static project's web.xml?

I found a solution here: http://wiki.metawerx.net/wiki/SecuringYourSiteWithContainerManagedSecurity

The page describes how to define your own META-INF/context.xml pointing to your own WEB-INF/users.xml. Unfortunately, the link to the users.xml file has to be absolute, and I do not want to make any assumptions on the OS/filesystem paths in my config files.

Here is my current WEB-INF/web.xml:

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app
    xmlns="http://java.sun.com/xml/ns/j2ee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_5.xsd"
    version="2.5">

    <display-name>SuperCoolTool</display-name>
    <description>What an awesome app!</description>

    <security-role>
        <role-name>manager</role-name>
    </security-role>
    <security-role>
        <role-name>keyuser</role-name>
    </security-role>

    <security-constraint>
        <web-resource-collection>
            <web-resource-name>
                Entire Application
            </web-resource-name>
            <url-pattern>/*</url-pattern>
        </web-resource-collection>
        <auth-constraint>
            <role-name>keyuser</role-name>
            <role-name>manager</role-name>
        </auth-constraint>
    </security-constraint>

    <login-config>
        <auth-method>BASIC</auth-method>
        <realm-name>Evaluation Area</realm-name>
    </login-config>

</web-app> 

An matching META-INF/context.xml would look like this:

<?xml version="1.0" encoding="UTF-8"?>
<Context>
    <Realm className="org.apache.catalina.realm.MemoryRealm"
           pathname="[PATH-TO-YOUR-WEBAPP]/WEB-INF/users.xml"/>
</Context>