I figured out how to splice in those resistors, based on some fine guidance from this forum (especially like to thank member phattyduck), and the high beam indicator light is working perfectly now.
Wasn't very difficult. Used 2 resistors (100 ohm 5W) wired in parallel (these replace the 2 missing halogen bulbs that the car is looking for). Shunted them between the ground wire and the low-beam wire (yes, you tap into the low-beam circuit because the indicator actually detects when the low beams are off, not when the high beams are on, or something like that. Not an EE myself, so can't really explain it correctly.)
I decided to make up a whole new extension harness. In case this didn't work, or caused other problems, I wanted it easily removable. Just ordered some 9004 connectors along with the resistors, and assembled it all. It looks a lot more complicated than it really is, but here's a pic (the load resistors look like they're in series, but they're actually wired in parallel):
Resistor Harness - 1 by
kpkina, on Flickr
(I even added a socket connector onto the high beam circuit in case I ever need to power extra driving lights or whatever.)