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

CSS:

    <style>
        html .RadButton.RadButton_Glow.RadButton.rbDisabled,
        html .RadButton.RadButton_Glow.RadButton.rbDisabled:hover,
        html .RadButton.RadButton_Glow.RadButton.rbDisabled:active,
        html .RadButton.RadButton_Glow.RadButton.rbDisabled:focus,
        html .RadButton.RadButton_Silk.RadButton.rbDisabled,
        html .RadButton.RadButton_Silk.RadButton.rbDisabled:hover,
        html .RadButton.RadButton_Silk.RadButton.rbDisabled:active,
        html .RadButton.RadButton_Silk.RadButton.rbDisabled:focus,
        html .RadButton.RadButton_Metro.RadButton.rbDisabled,
        html .RadButton.RadButton_Metro.RadButton.rbDisabled:hover,
        html .RadButton.RadButton_Metro.RadButton.rbDisabled:active,
        html .RadButton.RadButton_Metro.RadButton.rbDisabled:focus {
            background-color: transparent;
            background-image: none;
            border: none;
            box-shadow: none;
        }
    </style>

ASPX:

        <telerik:RadSkinManager ID="RadSkinManager1" runat="server" ShowChooser="true" Skin="Glow"></telerik:RadSkinManager>
        <telerik:RadButton ID="RadButton1" runat="server" ToggleType="Radio" ButtonType="ToggleButton" Checked="true" Enabled="true" />
        <telerik:RadButton ID="RadButton2" runat="server" ToggleType="Radio" ButtonType="ToggleButton" Checked="true" Enabled="false" />
        <telerik:RadButton ID="RadButton3" runat="server" ToggleType="Radio" ButtonType="ToggleButton" Checked="false" Enabled="true" />
        <telerik:RadButton ID="RadButton4" runat="server" ToggleType="Radio" ButtonType="ToggleButton" Checked="false" Enabled="false" />