I don't think you could hope for much more than 4-5 feet of height from an ornamental grass in a container that size, and you probably couldn't keep it outdoors in the winter. Your zone 6 winter feels like zone 4 to a container plant.

You might consider a trellis, with a fast growing annual flowering vine.

Darn, I was worried it wouldn't work! An annual vine on a trellis is a good idea. Thanks for the help, I appreciate it.

I'd look at bamboo - you'll get the height, it's evergreen and quite safe when planted in a container, even the running type. Most of the Phyllostachys (aka golden bamboo) are hardy to zone 4, the Fargesias (clumping bamboo) to at least zone 5. When grown in a container, bamboo only achieves about half its inground height so plan accordingly.

Here is a link that might be useful: Michigan bamboo