What i don't like about the SetGadgetFilter is, that it uses the gadget context field, which is of enormous help and may the reason it was not chosen. Best would be if the context field is completely free and another one is only used internally.
Unfortunately, the context field in TGadget is there specifically for storing the parameter passed by SetGadgetFilter(). Perhaps you need a new GadgetExtra()/SetGadgetExtra() function pair? ;-)