the only time you need to cover is when you are going to have frost or freeze. for the barrier you can leave it year round, it would have to get real cold to effect the roots.

There isn't a whole lot that is feasable for your situation other than to do as terrybull suggests when frosts are predicted to save what is left of the season. In order to keep the delicate blossoms from dropping which appears to be your goal, you could set up a temporary greenhouse over your plants but then you enter into a dozen more problems that may even be worse for your plants.

This subject (season extension) has led me to grow all of my tomatoes in high tunnels this year. If your weather conditions are as you describe them you might want to look into growing your tomatoes by a means that allows you more control of the temperatures that plants are subjected to.
There is occasionally interesting discussion on the "Greenhouses" forum if you are interested.