For the time being the property can be set through the kendoWidget:
ASPX:
		<script>
			function pageLoad() {
				$ = $telerik.$;
				var kendoWidget = $find("<%=PieChart1.ClientID%>")._chartObject;
				kendoWidget.options.series[0].labels.visible = function (point) {
					if (point.value < 5) {
						return false;
					} else {
						return true;
					};
				}
				kendoWidget.redraw();
			}
		</script>

		<telerik:RadHtmlChart runat="server" ID="PieChart1" Transitions="true" Width="450px" Height="450px">
			<PlotArea>
				<Series>
					<telerik:DonutSeries StartAngle="90">
						<LabelsAppearance Position="OutsideEnd" DataFormatString="{0} %" />
						<TooltipsAppearance DataFormatString="{0} %" />
						<SeriesItems>
							<telerik:PieSeriesItem BackgroundColor="Purple" Exploded="true" Name="Internet Explorer" Y="18.3" />
							<telerik:PieSeriesItem BackgroundColor="Orange" Exploded="false" Name="Firefox" Y="35.8" />
							<telerik:PieSeriesItem BackgroundColor="Green" Exploded="false" Name="Chrome" Y="38.3" />
							<telerik:PieSeriesItem BackgroundColor="Blue" Exploded="false" Name="Safari" Y="4.5" />
							<telerik:PieSeriesItem BackgroundColor="Red" Exploded="false" Name="Opera" Y="2.3" />
						</SeriesItems>
					</telerik:DonutSeries>
				</Series>
			</PlotArea>
			<ChartTitle Text="Browser Usage for April 2012">
			</ChartTitle>
		</telerik:RadHtmlChart>