Only a limited number of basic XPath filters are implemented in the Reportable REST API. This is how you can apply further filters on the returned data - this code returns all team members with a specific role in a team area:

  var getTeamMembersWithRole = function(team, role) {
    if (!team || !role) {
      console.log('getTeamMembersWithRole: Error! team = ' + team + ', role = '
          + role);
      return null;
    }

    var xhrArgs = {
      url : '/ccm/rpt/repository/foundation?fields=foundation/teamArea[itemId='
          + team
          + ']/roleAssignments/(contributor/itemId|contributorRoles/name)',
      headers : {
        'Accept' : 'application/xml'
      },
      handleAs : 'xml'
    };

    return jazzClient.xhrGet(xhrArgs).then(
        function(data) {
          var memberIds = data.evaluate(
              'foundation/teamArea/roleAssignments[contributorRoles/name/text()="'
                  + role + '"]/contributor/itemId', data, null,
              XPathResult.ANY_TYPE, null);

          var members = 0 || [];
          var id = memberIds.iterateNext();

          while (id !== null) {
            members.push(id.textContent);
            id = memberIds.iterateNext();
          }

          return members;
        });
  };