I have a similar plan, but I have a BCS-460 and a 16 output expansion board that Im going to use, I have 4 Temp inputs to use with the BCS when I'm not brewing so the idea is the temp inputs can moniter fermentations temp, the BCS will control a 3-way Solenoid valve
that diverts the coolant from the cooling loop to the cooling coils around the fermenter, and a SSR that controls a brew belt for heating for each fermenter. Right now I have one, but Id like to expand to 4. As for the DC control, I was looking at this 8 relay 12vdc
board from the brewtroller's site.
I think if you did not want to go the BCS route Im sure you could set something up from brewtroller that will work.