You don't need to implement a whole new editor. Simply attach to the EditorCreated event of the RadDataFilter. In the event handler check the name of the property and/or whether the editor is a MaskedTextBox. In case you detect that this is the editor that you want to configure, cast it to MaskedTextBox and then you can set any kind of Mask that you need.
I hope this helps. Let me know if there are problems.
All the best,
the Telerik team
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