Even if you cannot heat your greenhouse, there is a lot you can do to prevent freezing. On the other hand, though, there is a point after which effort is useless without a heat source. If it is just "pop" freezes, as you call them, and not generally not going below for more than a couple hours, there are several ideas you might like to try:
Frost Patrol® Plant Protector is a food-plant safe product I've used in the past, on late season tomatoes and peppers. You spray it over the plant, which absorbs it and put it to work within 24 hrs. The spray basically causes a change within the plant, which lowers the temperature at which the plants cells become damaged by 1 or 2 degrees. Spray it 48 hours before you expect frost (maybe every other day for you).
Lightweight cotton bed sheets are insulating and catch frost well. I've used these in emergencies where a sudden unforcast frost slips up on me and my row cloths are on the row crops. They are also nice because they lay fast and stay where you put them.
In an emergency, if you have a long enough electric cable to get power into the greenhouse, you can try using a small inexpensive electric heater. I know a guy who did this, and he said you have to distribute the air quickly so you don't overheat the plants. He placed the heater directly behind a large fan, and aimed that at the ceiling. The entire thing was put on a small timer and set to go 15 minutes to the hour.
As michelle said, high wattage incandescent bulbs will produce some heat. Do not install these within 3' of the plants, or damage may occur from inconsistent temperatures.
Don't know if this is an option, but I knew someone who would put a slow cooker full of water in their greenhouse on some nights, and simmer it all night.
You can use a combination of these if you feel it would be helpful. Growing tomatoes, sweet potatoes, and beans in an unheated greenhouse in your climate is something of a gamble. As long as you catch each freeze as it comes, though, you'll probably pull through. Also, the healthier a plant is, the better it will cope with cold stress. Keeping your plants in top condition (which it sounds like you're doing) is something to keep in mind. Also, you mentioned being out of row cloth. That is something you can fix right away. Row cloth is something you never can get too much of (unless you're a kleptomaniac).
