Here's the way I was able to git-er-done... I was using the RadMaskedTextBox but Telerik says that they are deprecating it. Instead, I used an asp:TextBox and the RadInputManager. Here's the code if you need it.
<asp:TextBox ID="txtSSN" runat="server"
requiredrule='<%#required("Social_Security_Number_Or_Federal_Identification_Number")%>'
onkeyup="ValidateTBNew()"></asp:TextBox>
and
<telerik:RadInputManager ID="RadInputManager1" runat="server">
<telerik:MaskedTextBoxSetting Mask="###-##-####" SelectionOnFocus="SelectAll">
<TargetControls>
<telerik:TargetInput ControlID="txtSSN" />
</TargetControls>
</telerik:MaskedTextBoxSetting>
</telerik:RadInputManager>
This way, the TextBox correctly renders with my extra requiredrule attribute and the formatting of the SSN is done with the RadInputManager.