Jennifer, I did once keep a valued semp under grow lights overwinter. It was in a small pot enclosed in a plastic bag so it had a constant level of moisture for the winter. It survived but did not grow larger until it was placed outside in spring.
I would suggest some more experimentation outside. If you can find a location that remains in constant light shade, that might be the right place for them, especially if you can keep them from getting wet. In the summer, especially in the south, light shade likely still provides more light than an indoor grow light arrangement. Also, consider using a fan to run air across the plants (sheltered from rain and water, of course). That should help with evaporative cooling.
I believe that semp's rosette structure can also function as a dew collection mechanism. The upward pointing rosette leaves allow condensation (dew) to trickle down to the roots of the plants. When the weather is hot and humidity high, a drop in temps in the night causes the plants to water themselves. A fan with moving air should help keep moisture levels down to acceptable levels.
Lastly, if you do decide to water, best do it by hand to the spaces between clumps of plants. I know in my climate, when the weather goes into the nineties, the plants do need to be occasionally but lightly watered or they will dry up if in the sun. The plants in my current semp nursery are not in the sun, but rather in light shade most of the day.
I share a similar problem with you but mine is opposite. During the winter time, I struggle to keep my Rhipsalis collection alive in an environment where the humidity is much too low for them. I always lose a few.
