Edit in JSFiddle

// Visualize: Customize report's DOM before render  (5.6)

visualize({
    auth: {
        name: "jasperadmin",
        password: "jasperadmin",
        organization: "organization_1"
    }
}, function (v) {
    //enable report chooser
    $(':disabled').prop('disabled', false);

    //render report from provided resource
    startReport();

    $("#selected_resource").change(startReport);
    
    function startReport () {
        //clean container
        $("#container").html("");
        //render report from another resource
        v("#container").report({
            resource: $("#selected_resource").val(),
            events:{  
                beforeRender: function(el){
                    //find all spans
                    $(el).find(".jrPage td span")
                         .each(function(i, e){
                               // make them red
                               $(e).css("color","red")
                                   .attr("data-my-attr", "test");
                          });                     
                    console.log($(el).find(".jrPage").html());
                }
            }
        });
    };
});

<!-- Visualize: Customize report's DOM before render  (5.6)
  -->

<script type='text/javascript' src="http://localhost:8080/jasperserver-pro/client/visualize.js"></script>
<script src="http://code.jquery.com/jquery-2.1.0.js"></script>

<select id="selected_resource" disabled="true" name="report">
    <option value="/public/Samples/Reports/1._Geographic_Results_by_Segment_Report">1._Geographic_Results_by_Segment_Report</option>
    <option value="/public/Samples/Reports/2_Sales_Mix_by_Demographic_Report">2_Sales_Mix_by_Demographic_Report</option>
    <option value="/public/Samples/Reports/3_Store_Segment_Performance_Report">3_Store_Segment_Performance_Report</option>
    <option value="/public/Samples/Reports/04._Product_Results_by_Store_Type_Report">04._Product_Results_by_Store_Type_Report</option>
    <option selected="selected" value="/public/Samples/Reports/AllAccounts">AllAccounts</option>
    <option value="/public/Samples/Reports/5g.AccountsReport">5g.AccountsReport</option>
    <option value="/public/Samples/Reports/06g.ProfitDetailReport">06g.ProfitDetailReport</option>
    <option value="/public/Samples/Reports/ProfitDetailReport">ProfitDetailReport</option>
    <option value="/public/Samples/Reports/07g.RevenueDetailReport">07g.RevenueDetailReport</option>
    <option value="/public/Samples/Reports/RevenueDetailReport">RevenueDetailReport</option>
    <option value="/public/Samples/Reports/08g.UnitSalesDetailReport">08g.UnitSalesDetailReport</option>
    <option value="/public/Samples/Reports/08.UnitSalesDetailReport">08.UnitSalesDetailReport</option>
    <option value="/public/Samples/Reports/9.CustomerDetailReport">9.CustomerDetailReport</option>
    <option value="/public/Samples/Reports/9g.CustomerDetailReport">9g.CustomerDetailReport</option>
    <option value="/public/Samples/Reports/10g.PerformanceByStoreType">10g.PerformanceByStoreType</option>
    <option value="/public/Samples/Reports/State_Performance">State_Performance</option>
    <option value="/public/Samples/Reports/11g.SalesByMonthReport">11g.SalesByMonthReport</option>
    <option value="/public/Samples/Reports/SalesByMonthReport">SalesByMonthReport</option>
    <option value="/public/Samples/Reports/12g.PromotionDetailsReport">/public/Samples/Reports/12g.PromotionDetailsReport</option>
    <option value="/public/Samples/Reports/PromotionDetailsReport">PromotionDetailsReport</option>
    <option value="/public/Samples/Reports/13g.TopFivesReport">13g.TopFivesReport</option>
    <option value="/public/Samples/Reports/14.PerformanceSummary">14.PerformanceSummary (including fusion chart)</option>
    <option value=""></option>
    <option value=""></option>
    <option value=""></option>
    <option value=""></option>
    <option value=""></option>
    <option value=""></option>
    <option value=""></option>
    <option value=""></option>
    <option value=""></option>
    <option value=""></option>
    <option value=""></option>
</select>
<!--Provide container to render your visualization-->
<div id="container"></div>

.control {
    display: block;
}
.control.input.text > .wrap {
    display:block;
}
.control > input {
    width: 360px;
}
.error, .logout {
    color: red;
}
.success {
    color: green;
}
.message {
    display: none;
}