Steve,

My question (finally!) is: Does the lead from the light to the alternator,
play an active part in the energizing of the alternator field?

                  Yes
 Meaning,
if I interrupt this circuit, do I disable the alternator? Or, is the light
seperate from the charging-circuit part of the alternator, and I've done
something else to destroy the entire alternator?

                No

  Ok this is how GM built them. The post from that alternator
  goes directly to the battery. This leaves 2 other quick
  connect terminals. One goes to a 12v source that is switched
  with the ignition switch. The other must go to a resistance
  and then to ground. The resistance in most GM car is a #194
  light bulb!!!!
          Who knows what the PO did with the existing VW light
     there??? Good luck, keep us  up to date

                                     ERic ZEno
                                     86 GL
                                     86 Syncro (still leaking anti-freeze)
                                     72 240Z
                                  1936 Chrysler (with GM alternator)