I'm not sure if this helps you in your climate, but I am in the SF Bay Area, and we occasionally get weather in the winter that threatens or kills bananas, and certainly could freeze its fruit. One thing I've done that helped me fruit a late blooming banana is to:
1. Put 3 35-gallon black covered trashcans full of water around the trunks. Water is very good at holding heat, so the trashcans absorb heat in the day and radiate it at night, protecting the trunks. This probably adds enough protection that the plants can start growing as soon as things start warming up in March, which helps them fruit in enough time to ripen.
2. Cover the whole thing (fruit stalk, trashcans and plant) with a Remay blanket. Water and air can get to the plant, but Remay (or a similar synthetic fabric) probably adds a few extra degrees to the coldest temp your tree can tolerate.
EDIT: One thing I forgot to mention is that it is a good idea to put a cup of bleach into the water, to prevent algae and mosquitoes from accumulating if the cover comes off. The other thing is, since you might want to move the cans around, I bought mine to be the wheeled kind, which makes that a lot easier.
I have managed to fruit a few of the more cold tolerant bananas here in San Jose, even though our climate isn't good for it. I have had luck with and fruited California Gold, Sweetheart, and Dwarf Orinocco, and have young plants of Blue Java, Manzano, Goldfinger, and a few others that have survived winters but have not yet fruited.
Best of luck,
Lisa
