George - You are indeed correct about I squared R heating of the stator not being an issue!
My concern rather is the rotating field winding. Depending on the alternator electrical characteristics, the field could draw as much a 4x design current for the alternator to deliver 48 volts. And this would mean 16 TIMES the heat developed in the winding. Whether or not it would be a problem is uncertain.
As for where to find a 48 volt regulator, suggest paying a visit to your neighborhood electronics nut. If he's any good it should be a relatively simple matter for him to cob up a 48 volt PWM field control regulator.