I'm not sure I fully understand "sub irrigated planters", but I think the answer is, mostly, yes.
There are folkz here who sink their reservoir in the ground so the earth acts as a heat sink to draw the nutrient temperature down thus eliminating the need to cool their nutrient solution. Note, however, there are many factors that will dictate how well this works. some are:
reservoir size, type of hydro system used, number of plants supported, whether the supply and return lines are buried or insulated.
I'm sure there are more. I don't sink mine so I haven't dabbled that much with the concept.

Thank you very much. I'm a total newb at anything more than 'water the potted plant daily in summer'.

No problem. If you have more questions, just ask away.
Some people freeze water in two liter bottles then plop that in the reservoir to keep the temperatures down, so that's another option. It might be a better choice so you can make sure you like hydro before you commit to digging a big hole in your backyard. :D