For the time being the following workaround can be used:    

Category Axis:

JavaScript:

  <script>
            function pageLoad() {
                var chart = $find("<%=RadHtmlChart1.ClientID%>");
                chart._chartObject.options.categoryAxis.labels.template = "Item: #=value#";
                chart.repaint();
            }
        </script>

ASPX:

       <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="300">
            <PlotArea>
                <Series>
                    <telerik:ColumnSeries Name="series 1">
                        <SeriesItems>
                            <telerik:CategorySeriesItem Y="30" />
                            <telerik:CategorySeriesItem Y="10" />
                            <telerik:CategorySeriesItem Y="25" />
                            <telerik:CategorySeriesItem Y="20" />
                        </SeriesItems>
                    </telerik:ColumnSeries>
                </Series>
                <XAxis>
                    <Items>
                        <telerik:AxisItem LabelText="1" />
                        <telerik:AxisItem LabelText="2" />
                        <telerik:AxisItem LabelText="3" />
                        <telerik:AxisItem LabelText="4" />
                    </Items>
                </XAxis>
            </PlotArea>
        </telerik:RadHtmlChart>

Numeric Axis:
        <script>
            function changeValues(value) {
                var newValue;
                switch (value) {
                    case 1:
                        newValue = "AA";
                        break;
                    case 2:
                        newValue = "BB";
                        break;
                    case 3:
                        newValue = "CC";
                        break;
                    case 4:
                        newValue = "DD";
                        break;
                    default:
                        newValue = value;
                }
                return newValue;
            }
            function pageLoad() {
                var chart = $find("<%=RadHtmlChart1.ClientID%>");
                chart._chartObject.options.xAxis.labels.template = "#=changeValues(value)#";
                chart.repaint();
            }
        </script>
        <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600" Height="300">
            <PlotArea>
                <Series>
                    <telerik:ScatterLineSeries Name="series 1">
                        <SeriesItems>
                            <telerik:ScatterSeriesItem Y="30" X="1" />
                            <telerik:ScatterSeriesItem Y="10" X="2" />
                            <telerik:ScatterSeriesItem Y="25" X="3" />
                            <telerik:ScatterSeriesItem Y="20" X="4" />
                        </SeriesItems>
                    </telerik:ScatterLineSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>