It is a great idea. In fact, you can probably use any netbook or notebook cart that has enough outlets configured for the Apple plug. The Apple AC adapters draw 10 watts, so, even allowing for some inefficiencies, 40 iPads would be the equivalent to around four 100 watt light bulbs, which is no problem. You should not need a timer or a so-called "Smart Brain." It is just not worth the price.
There are other charging carts that are compact and very secure. Security is really important. Someone recently said putting a cart full of iPads in a classroom is like putting in an ATM with an open card. My school got some great security carts from Datamation Systems and they are VERY secure. They have versions that store and charge 16, 20, 38, and 40 iPads.
Also, make sure you get something that has adjustable slots or slots large enough for the cases you use now or may use some day in the future.
Hope this helps.