Its always nice to make changes using a custom module instead of editing the core modules. That way, you will be able to debug your issues. Here is what works for me:

 /custom_module/static/src/js/subst.js   

function subst() {
    var vars = {};
    var x = document.location.search.substring(1).split('&');
    for (var i in x) {
        var z = x[i].split('=', 2);
        vars[z[0]] = unescape(z[1]);
    }
    var x=['frompage', 'topage', 'page', 'webpage', 'section', 'subsection', 'subsubsection'];
    for (var i in x) {
        var y = document.getElementsByClassName(x[i]);
        for (var j=0; j<y.length; ++j)
            y[j].textContent = vars[x[i]];
    }
    var operations = {
        'last-page': function (elt) {
            elt.style.visibility = (vars.page === vars.topage) ? "visible" : "hidden";
        },
		'first-page': function (elt) {
            elt.style.display = (vars.page == "1") ? "initial" : "none";
        },
		'other-page': function (elt) {
            elt.style.display = (vars.page != "1") ? "initial" : "none";
        },
    };
    for (var klass in operations) {
        var y = document.getElementsByClassName(klass);
        for (var j=0; j<y.length; ++j)
            operations[klass](y[j]);
    }
}

XML to replace enque subst.js using custom_module:

  <template id="minimal_layout" inherit_id="report.minimal_layout">    
    <xpath expr="//script" position="replace">
      <script src='/custom_module/static/src/js/subst.js'></script>
    </xpath>
  </template>

You can then use class="first-page" and class="other-page" in your views. 

PS. Some older versions of the report module will enqueue the subst.js differently. Refer to that if you get an "id not found in view" error.