Implement JavaConverter for a rank 2 type

[jneira]

Hi, i want to wrap this type (that i cant change):

type Normalizer = forall s. Expr s a -> Maybe (Expr s a)

with this java Function-like:

data JNormalizer s a = JNormalizer (@org.dhall.eta.core.Normalizer s a)
  deriving Class

foreign import java unsafe "apply" applyJNormalizer
  :: (s <: Object, a <: Object, t <: JExpr s a, r <: JExpr s a)
  => (t -> Java (JNormalizer s a) (JOptional r))

given i have instances of JavaConverter for all Dhall.Expr JExpr, (Maybe a) (JOptional ja), etc

Afaik you could not "use" the type s' and you cant add the constraint JavaConverter s js after the rank 2 type Normalizer definition.
Is there any workaround (changing the definition of JNormalizer or JavaConverter f. e.)?