That would certainly work, and so would several rare earth magnets placed around in there, instead of the bobbin winding. Whatever you decide, once the steel has magnetically saturated, there is no further increase in alternator output. So there is no way to lower the rpm requirement for higher required voltage, at least not without rewinding the star output winding.
The disadvantage of a PM rotor would be that the output voltage could not then be so easily controlled.
Something that will work really hard then pull the charging current right back to zero once full battery voltage is reached is an absolute necessity. Its pretty important not to boil the battery !
It could certainly be done with some kind of buck regulator on the output, but that would have to carry the full alternator output power, perhaps a couple of kilowatts. Its much more convenient to control the much smaller current in the field winding, the power required to do that is only roughly about fifty watts, not kilowatts.
Controlling the IC engine throttle opening is another possibility, but it is not a very nice way to do it.
And if something goes a little bit wrong, once again you might cook a battery.
The original automotive voltage regulator worked fine at 12v, it just needs something very similar to work in the exact same way, but designed to work at 48v.