Some people drill hole somewhere and spray in a coating and hope that it covers the entire backside.
Here's what I would probably do: Coat the middle of the backside of the piece with POR15 (don't coat the 1/2" edge, because you don't want it to be dragged into the weld). Sand and coat inside the opening as much as you can reach with POR15 (again, except for within 1/2" of where the weld will be.). Spray the 1/2" edges with a weld-thru primer (weldable primer that is high in zinc content). When you weld, the weld-thru primer will be pulled into the welds and will help protect them from corrosion. (Honda uses a crappy weld-thru primer to protect the back of panels. Get a good quality one, and the metal will be protected better than from the factory) When everything is done, coat the front surface with POR15.
That's how I would do it. Anyone else got any ideas? I've had a couple of sleepness nights pondering this little problem as well.