Okay, success! I am SO happy to be able to control whether or not the dome lights come on when I open the doors (or want to have the doors remain open for some reason). Yessss!
As it turned out, the nearby switch I was going to re-purpose was a momentary on (duh, how did I not remember that). Of course I could have replaced it with a regular switch, but I really didn't need/want that feature anyway - I was just doing it because it seemed easier to figure out. So I decided to forge ahead and try to figure out Jage's way.
So, what I ended up with is this:
1) No new switches, now the driver's side "button" on the stock dome light turns both lights on and off manually, and the passenger side "button" engages or dis-engages the "comes on when the door opens" part. I don't think I'll use that too often, but I wanted it there just in case.
2) Both lights now come on and go off together, but that's fine with me, especially since they can be "aimed," so they could both be aimed to passenger side if desired.
If someone else wants to try it, I can say how mine was set up, which seems to basically agree with Woodbee's. There is also the option to just eliminate the door feature and maintain individual light control (easiest), or go with Woodbee's setup (adds a switch, but retains both door option and individual light control).
There were essentially three wires coming in on mine.
1) White wire (ground)
2) Black wire (this is the "door control" wire).
3) Pair of red wires (electrically together) which control the manual switch buttons on the lights. This was one green wire on Woodbee's rig, but otherwise ours were the same. I think mine might have been a pair because there was also an illuminated rear-view mirror, but in any case, they were like one wire as they came into the dome light.
a) So, by simply cutting the black incoming wire, the "door control" is taken away (ha, take that!). If you simply leave it at that, I believe you will have two manual dome lights, each controlled by their own button switch - but no option to have doors activate lights.
b) By cutting that same black wire and then taking the cut ends to a separate (new) switch, you get Woodbee's setup. That's each light still controlled by its own button, and the new switch activates or de-activates the door control.
c) What I did....
1) Take the two larger black wires that come off the back of the bulbs and go to the small red wires on each switch, and instead join both of them to the small red wire on the driver's side switch (and no longer to the small red wire on the passenger side switch).
2) Cut the small black wire that went from red main incoming wire (from the ceiling) to the passenger side switch. Both ends will be taped off (or however you want to close them off) and not used.
3) Cut the thin black wire that goes from the black incoming ceiling wire to the driver's side switch. The ceiling incoming black wire gets reconnected (or stays connected) to the thin black wire on the passenger side switch (we reconnected just for tidiness of joints). The thin black wire part that is still on the driver's side switch gets connected to the red wire on the passenger side switch.
Notes:
i) The orange wires are new pieces added for length convenience. I used it because it was 18 gauge and that was best for the crimps when bridging between the various sizes.
ii) In case it helps to clarify the photo, the clearish butt connectors that look a bit like wire nuts were original connections, and the pink/blue heat shrink butts are the new ones.
iii) I did not become a wizard overnight; a friend and I figured it out together.
iv) Of course I can't know how anyone else's wiring is set up - this seems to work for my dome light.
Viva