the big risk is uneven moisture - if they get a bit dryish and you get a heavy rain, they will crack open and spoil. If you can keep the soil around them consistently wet, read almost soggy, they should keep going another month or so. I believe you can pick them and store them in the shade, even if its still hot out. I'm a bit unsure on the heat issue, but I have neighbors who store cabbage in their garage for months. but they pick it in late Sept.

My DD9 brought home two cabbage plants for a 'Giant Cabbage Contest' in March. I grew them in the greenhouse for a while, and then set them out in different locations in the garden - one by the greenhouse split with a recent rain, the runoff from the roof was too much. The other, I put out at the down-hill foot of my irrigated field / garden, where it stays soggy all summer. That one, the head is about 18 " in dia, and it started growing smaller cabbages to the side. I broke off 3 of them this morning, each the size of my fist.

We wait until the the frost hits hard, the first couple of frosts do not effect cabbages as they are very hardy
In Alaska we have to wait until the last second since our growing season is so short

Here is a link that might be useful: Alaska Gardening Information