You hit the nail on the head, Paul.  The problem with the alternator is the dynamic response of the regulator driving the inductance of the field.  With some regulators the system will be unstable without a battery, but as you say, it is certainly possible to build a regulator that will work.
 
Gary

From Paul:
I guess the alternator's field inductance =
could drive a slow-responding regulator into the heebie-jeebies!