Little late to the party here, but have you ever grown Mizuna? It's a mild Japanese type of mustard green. I think it tastes like sesame, a bit.
Anyway, the last few years we lived in Utah I'd plant it late into the summer, and it would last a very long time, sometimes even under the snow out in the garden!
There's also Corn Salad, can't for the life of me remember what the botanical name for it is, but it survived frost here a couple of years ago and came up smiling.
I'm sure you plant lettuce late in the summer/fall for late greens in the cool weather, right? I wonder if you could bring pots of those into the house?
