If you're worried about your bike, a chunk of metals of varied corrosion types and reaction rates, corroding due to exposure to salt spray or similar, cathodic protection is not the way to go. Protective coating is the way to go. If you're worried about corrosion at points where metals that are widely separated on the galvanic scale are in contact, carrying current and exposed to salt air or spray, cathodic protection using an applied DC voltage could possibly help. But a solution of much less cost and complexity is to use a copper wire (insulated) as a return path, rather than the bike frame. At least then if there is corrosion, it will involve a cheap wire and not an expensive frame.
Cathodic protection is not free. if you apply a voltage across two metals and a closed circuit exists, current will flow. The generator that applies the voltage must also provide current, which means power will be deliverd by that generator. If the source fo power to the DC generator is the lighting dynamo, then the ultimate source of power is your legs. It's important to know what voltage is needed and what current can be expected to flow. In large facilities using cathodic protection to prevent corrosion of the grounding system, numerous kilowatts are allocated when budgeting the power.
Again, I think the best solution is to keep the return current out of the frame: use a return wire, and make sure the frame is not capable of carrying return current. Mount the generator or the lights so that the return or "mass" terminals are insulated from the frame.
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Ken Freeman
Ann Arbor, MI USA