Yes, make a dummy style resource where all class names are marked @external (
http://www.gwtproject.org/doc/latest/DevGuideClientBundle.html#External_and_legacy_scopes) and override the methods of CellTable.Style to annotate them with @ClassName to match the CSS class names you used in the stylesheet.
(technically, you could also probably pass your own implementation of CellTable.Resources where the CellTable.Style methods return constants and ensureInjected() is a no-op)