MyPackagePage.page

<apex:page>

    <apex:includeScript value="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js" />

    <div>
        <h1>This is the Managed Package page</h1>
    </div>

    <div id="baseUrlLink" style="display: none;">
        <apex:outputLink value="/">BASE URL</apex:outputLink>
    </div>

    <iframe id="customPageIFrame" src="UserCustomPage" width="500" height="500"/>

    <script>
        function updateIFrameSrc() {
            var baseURL = jQuery('div#baseUrlLink > a').attr('href');
            var customPageUrl = baseURL + 'apex/UserCustomPage';
            jQuery('iframe#customPageIFrame').attr('src', customPageUrl);
        }

        jQuery(document).ready(function() {
            updateIFrameSrc();
        });
    </script>
    
</apex:page>