If I were so motivated to do this internally, I would disconnect the positive side of the field and hook it up to any convient battery+ through a switch. Then you only have to add one wire to your alternator.
Looks like the field winding gets grounded by the regulator to energize the field winding. The two wires going to the ignition and idot light are used to operate the idot light so that it doesn't blink when the engine is being shut down and so it does light when the ignition key is turned on with the engine NOT started yet - otherwise it would be on all the time? Nowyou inderstand why I think it would be easier to supliment the battery power from another power source and leave the existing charging circuit alone.