The current itself is flowing in the coolent so the path is not accessable. (inside your tubes and coolant passages)
Electrical bonding of the metals can help reduce the potentials developed, however there is a chemical process involved as well.
Zinc anodes will help, but they're most effective if you use several scattered across the coolant path. Basically they promote electron flow to the
zinc, but you have many different paths and flows occuring. I'd suggest one at the radiator and one or more at the block.
This will sound nuts but kerosine is a great coolant that's been used in exteremely cold enviroments. Obviously has it's other drawbacks though.
You can use no-lox for electrical connections as well as anti seize. I think about the only metal aluminum gets along with is brass / copper.