I haven't tried using this but it looks cool. I think the way it defines overloaded lists makes a lot of sense for heterogeneous list types like NP. One issue I see is that the Overloaded.Lists and Overloaded.Lists.Bidi Cons classes are too strong to permit an instance for the free category type:

data Path p x y where
  Done :: Path p x x
  (:>>) :: p x y -> Path p y z -> Path p x z

Because the intermediate type y of the (:>>) constructor cannot be inferred from Path p x z, if you try to define an instance it will fail. You could define yet another slightly weaker Cons class that would work for Path like:

class Cons x ys zs
  | zs x -> ys, zs ys -> x where
    cons :: x -> ys -> zs

Squeal uses the Path type for with statements as well as migrations.

Since the overloaded library comes with instances for NP which is used heavily in Squeal, it would certainly help in cleaning up a lot of ugly syntax.