Forgot to mention there is a third option which you alluded to Peter, fitting extra diode(s) into the field circuit after diode trio, although some experimentation may be needed to get desired regulating voltage increase.
See article with rationale
here.
Note many car alternators with internal regulation do limit to 14.4 volts. I suspect this is because battery replacements are near universally sealed, maintenance free type of lead/calcium construction. The figure is likely a pitch to cover the fact that only limited reserves of electrolyte are stored within and concerns over evaporation of same.
The addition of calcium in plate construction allows greater rigidity and less gassing at higher charge voltages, safely allowing charge voltages in excess of 15 volts though. I have charged large battery banks of sealed lead/calcium throughout their life at max of 14.8 volts allowing greater charge density and reducing the ageing sulphation process. Out of interest I tore off the tops of a couple before re-cycling and found plenty of remaining electrolyte still covering plates.
