My guess is their ovens set from 150 up. Some do. Also I bet they're using kettles for mash tuns. Most coolers wouldn't fit very well in an oven.
My recommendation is to take the mash temp of the recipe and up it by 2 degrees. That's what I'm going to be doing. If it's a lower mash temp, maybe ramp it up 3-4 degrees. You want to avoid sitting in the high 140's without having denatured beta amylase. Otherwise you'll end up with super dry beer like my 1.001 Pale Ale.
I'd say the lowest I'd mash overnight, without much crystal or dextrine malt, is 155. That will still likely result in a dry-ish beer.
We'll see where my 156 mash (oktoberFAST) comes out. I'm guessing it will be slightly more dry than I wanted, but with 10% crystal and 40% Munich I'm hoping there's still some malt character.
I'm using a 10 gallon cooler for my tun, and am thinking about trying this on my next brew. My question has to do with mashout. Are you doing any sparging after the overnight mash? In my head, I'd imagine you'd lauter in the morning, and then heat the sparge water to 170, hold for 10-ish minutes, and then drain to the kettle for the boil like normal. Is there any reason you wouldnt be able to sparge as normal? I dont imagine there would be. Are you also doing a 60 minute boil, or do you boil for longer?