Ooo! A challenge. I have no idea what an eastern exposure in Seattle is like, but, assuming it means a little morning sun, and not much else, I'd pick Deschampsia first, and Chasmanthium next.

Deschampsia will give you more of a short and wide profile.
Chasmanthium will be taller and more narrow. Deschampsia looks more like grass, and Chasmanthium more like Bamboo.

I haven't grown Molinia, but 'Morning Light' needs more sun, and would require a really big container as it ages. C. brachytricha is new for me, having just started some from seed, so I can't comment on it yet.

An unobstructed eastern exposure in this area typically allows for sun in summer until midday, later if out of the shadow of the structure.

I'm not sure how you interpret "specimen", but to me that means something with significant presence and preferably evergreen. I'd eliminate the molinea just because it is a short, stubby clumper that only achieves height from the late summer infloresence (but it's a great grass otherwise). You might want to consider one of the carex - most are partially to fully shade tolerant here, even the New Zealand species, and are evergreen. Carex secta makes an impressive statement as does Carex testacea, which turns interesting shades of copper and orange with cold weather. If the pot size is smaller, Carex morrowii cultivars like 'Ice Dance' or 'Aureovariegata' would work well, as would C. oshimensis 'Evergold' or C. phyllocephala 'Sparkler', which is a fascinating grass that has a distinctly bamboo-like appearance. Bamboos are also a good choice as they easily tolerate part shade here, are well-controlled in a contained environment and will provide you evergreen height. I also grow a number of New Zealand flax (Phormium ctvs.) in both east and north facing situations in my garden and they thrive in less than full sun conditions. Lots of extremely attractive and colorful choices with the flaxes and they would definitely be considered specimen quality.

You will need to keep on top of the watering in our very dry summers with both the carex or the bamboos - neither appreciates droughty conditions. The phormiums are a bit more tolerant.

Wow! Excellent suggestions from Donn & GardenGal

I can only add a couple of things....
The Molinia really does do much better with more sun. I do have one growing in the conditions you suggest and it has remained relatively small and insignificant for the past 8 years. It grows but doesn't do much in this situation.

Calamagrostis brachytricha is a lovely grass and will take much more shade than C. a. 'Karl foerster'. But it really doesn't have much presence until it blooms. I am afraid, if you have it as a specimen in a container people will .... wonder :O)

The Miscanthus 'Morning Light' will do fine in 1/2 day sun but will require quite a large container and will not get as tall as it might in full sun. Never the less, it does brighten up a shady spot.

I just reread your post & I see you say grasse(s) and container(s) plural .. I think that our answers were in response to the word 'specimen' and all the posters have been trying to come up with one all-round grass. So .. in reflextion, you might use any of your choices if they were to be put in a grouping. AND, since they will have the lovely back-drop of a plain wall, I would seriously look at grasses who's overall shapes that appeal to you. If viewed from a distance their silloettes could be quite striking.

Please post a picture when you get this area set up. It really could be quite lovely!