Well I went to pull the voltage regulator and/or alternator on the parts car today, and being that it's a 300D with turbo, it's not NEARLY as easy to get to as the 240D. So for what it's worth, I put the charged battery back in the 240D and took voltage readings with the car running across the temporarily removed positive voltage cable and negative cable (to isolate the battery). I was getting ~14 volts, so the alternator was still working even with the old brushes and regulator.
My hunch is that the brushes in the voltage regualtor were starting to intermittently not work, so for $8.48 at the hardware store, I bought new brushes and resoldered them in place. After installation, I checked voltage readings again, as before isolating the battery, and was getting closer to 15 volts on the DMM.
(Pictures of the new brushes in the old voltage regulator attached.)
Thanks everybody for all the pointers, and as always the very helpful archival posts!
