This is a tough one. There are quite a few grasses, in a variety of small to medium sizes, which will do fine in shade or indirect light. Unfortunately, not as many are hardy to your zone, and most of them require moist fertile soil.

If you can satisfy the moist fertile soil issue, check some of these:

Calamagrostis arundinacea var. brachytricha
Carex conica
Carex glauca
Carex muskingumensis
Carex nigra
Carex pendula
Carex plantaginea
Chasmanthium latifolium
Deschampsia caespitosa
Deschampsia flexuosa
Elymus arenarius
Festuca gigantea
Festuca scoparia
Hystrix patula
Luzula nivea
Luzula sylvatica
Melica altissima
Panicum clandestinum
Phalaeiz arundinacea
Sesleria caerulea

A potential way to get more light into the area is to install some large white boulders. While they'll make some shade on the down-side, they'll reflect some light into the area.