I have read about but haven't tried setting up incandescent lights in cold frames (I don't have one but am seriously thinking I want one
). Some say ordinary light bulb, some say Chick incubator. I use the tiny holiday lights under and around them in my unheated garage to keep them warm -- sometimes the temp in there have plummeted to 20's -- I wrap my entire seed starting set up with plastic on those coldest nights.
Tomatoes can take 40's and probably survive upper 30's, peppers are said to shut down in lower 50's and suffer growth set backs in 40's so put them closer to whatever heat source you use. they can survive the cold better if they have bottom heat --especially the peppers.
A second layer of floating row covers inside the cold frame would protect them better -- I'm sure you know about dressing in layers