The method should be called after the text that should be copied to the clipboard is extracted. In its original implementation it should only set the text to the clipboard. You should be able to override it and replace the text or use different method for copying into the clipboard.

For example:
public class CustomMaskedInputControl : RadMaskedDateTimeInput
{
    protected override void SaveTextToClipboard(string text)
    {
        Clipboard.SetDataObject(text);
    }
}