In 4.4.5 we say:

Likewise, if a sub type overrides/implements a method originally defined in several parallel types of the hierarchy (e.g. two interfaces not extending each other, or a class and an interface not implemented by said class) and if that method's return value has been marked for cascading validation in one of the parallel types, no group conversion rule may be declared for that method's return value in the parallel types of the hierarchy.

Maybe this could be relaxed to say "... marked for cascading validation in more than one of the parallel types"? I think if there is only one method marked as cascaded, this could perform a conversion, too. Just a minor thing, though.