You might try Silky Thread grass (Nassella tenuissima). It's a good size for a window, looks nice all year (it just needs gently combed or raked out in the early spring), and can look sculptural in the right context. It looks *fabulous* when the wind blows. It's recommended more for the west than the east, but since you'll be growing in pots you could probably come closer to re-creating its preferred environment.

Here is a link that might be useful: Silky Thread grass