Have you considered using IDataErorInfo interface for a validation mechanism ? It's validation code will be triggered early (when the binding is initially evaluated) and the control will be loaded with red border. This will notify the user that this field is requried.
On the other hand vfalidating with DataAnnotations is not so suitable because the Validation Exception will be fired before the MaskedInput has loaded and this will break the application.
Let us know what you think of the approach with IDataErrorInfo.
On a side note, you can take a look at this great article for Validation in .Net.
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