Modifying brushless alternator for external regulator
In general, a brushless alternator would be two-in-one, comprising a main one and an exciter, axially side-by-side.

In the main alternator, the stator would be the armature and the rotor the field, whereas in the exciter, the stator would be the field and the rotor the armature.  This configuration would enable the exciter armature to feed the main field through a bridge rectifier, without the need for slip rings.

The exciter field winding would be fed by the main armature through a bridge rectifier and regulator.

The main rectifier block and regulator block would be separate sub-assemblies.
Should the regulator block have bolt terminals, it could be removed and its input and output cables brought out, to connect an external regulator.