Currently ClientTemplates are exposed only for the Series labels and tooltips. Improving this functionality will let you feed data for the legend from more than one column of the datasource as well as to format numbers and dates in the legend. The benefit will be essential for Pie and Donut series.

For the time being the following workaround can be used:
JavaScript:

    <script>
        function pageLoad() {
            var chart = $find("<%=RadHtmlChart1.ClientID%>");
            chart._chartObject.options.legend.labels.template = "#=kendo.format(\'{0:d}\',dataItem.SellDate)#";
            chart.repaint();
        }
    </script>

ASPX:

        <telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="640px" Height="480px">
            <PlotArea>
                <Series>
                    <telerik:PieSeries DataFieldY="SellQuantity" NameField="SellDate">
                    </telerik:PieSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>