I am cooking turkey dinner for 140 people at our church. My meal consist of turkey, stuffing balls, mashed potatoes & gravy, green beans, noodles in gravy, and cole slaw. I am not sure of the amounts of food I should cook and your advice would be very helpful.   Thank You  
You can indeed dehydrate cooked rice.  It is a lack of water activity that is one factor which increases the safety of foods, so no problem there. Rice is no more problematic than other foods. As you probably know, dehydrated foods are a staple for camping and backpacking. So, cook your rice at home, then place in a food dehydrator. Once on the trail, a 5 minute pre-soak, bring to a boil, cook for a minute, rest for a few minutes...you are good to go.  Edit:  With a potential concern for the growth of B. cereus in cooked rice, I would be more comfortable raising the temperature of the dehydrator to at least 60C/140F.  Higher temps for rice in your dehydrator are not a problem, it will just proceed more quickly.  Reference here. 