The functionality is currently exposed only by the x-axis of the sparkliens. It would be a good idea to be exposed by the other series types as well.

For the time being you can configure the cross hair through the kendo widget:
<script>
			function pageLoad() {
				$find('RadHtmlChart1').get_kendoWidget().setOptions({categoryAxis:{crosshair:{ color: "green", width: 2, visible: true}}});
			}
		</script>
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="400">
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Series 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="30" />
							<telerik:CategorySeriesItem Y="10" />
							<telerik:CategorySeriesItem Y="20" />
						</SeriesItems>
					</telerik:ColumnSeries>
					<telerik:LineSeries Name="Series 2">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="39" />
							<telerik:CategorySeriesItem Y="19" />
							<telerik:CategorySeriesItem Y="29" />
						</SeriesItems>
					</telerik:LineSeries>
				</Series>
				<XAxis>
					<LabelsAppearance RotationAngle="33"></LabelsAppearance>
					<Items>
						<telerik:AxisItem LabelText="Item 1" />
						<telerik:AxisItem LabelText="Item 2" />
						<telerik:AxisItem LabelText="Item 3" />
					</Items>
				</XAxis>
			</PlotArea>
		</telerik:RadHtmlChart>