For the time being the function can be set through the kendoWidget:
ASPX:
		<telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
		<script>
			function pageLoad() {
				var chart = $find("<%=RadHtmlChart1.ClientID%>");
				chart._chartObject.options.series[0].markers.type = function (point) {
					if (point.value > 2) {
						return "triangle";
					}
					else {
						return "circle";
					}
				};
				chart.repaint();
			}
		</script>
		<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="600px" Height="400px">
			<PlotArea>
				<Series>
					<telerik:LineSeries Name="Product 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="1" />
							<telerik:CategorySeriesItem Y="2" />
							<telerik:CategorySeriesItem Y="3" />
						</SeriesItems>
					</telerik:LineSeries>
				</Series>
				<XAxis>
					<Items>
						<telerik:AxisItem LabelText="1" />
						<telerik:AxisItem LabelText="2" />
						<telerik:AxisItem LabelText="3" />
					</Items>
				</XAxis>
			</PlotArea>
			<ChartTitle Text="Product sales for 2011">
			</ChartTitle>
			<Legend>
				<Appearance Position="Bottom" />
			</Legend>
		</telerik:RadHtmlChart>