I have a spare aftermarket coil for my IT400 Yamaha. The IT400 did not have a battery, it is a pure capacitor discharge type set up. However, recently in trying to figure out what was wrong with my xl250 (6 volt for the accessories, capacitor discharge for the ignition) I plugged it in and it fired the plug beautifully. I'm not sure if that means its a 6 volt or not. I don't think it matters actually, but I am not sure.
Does your Harley stator/rotor have a circuit which is separate for the ignition? If so this would probably work. If the ignition is points and gets its current from the 6 volt battery, you have a different system.
Let me know.
Peace & Grease, Dennis