Today I woke up with an idea: to try and get someone else to do the mod for me. In fact, there are cellphone repair shops practically in every street corner around here, and these shops deal with much more minute/delicate electronics (the ones inside cellphones) so I was confident that they could do the job. Then I selected a nearby shop with good reputation from GoogleMaps and headed there.
The good news is, in the end IT WORKED! YAY! Finally I can backup my original firmware and config, and update my D3AA! 
The not so good news is, it demanded a LOT of effort. The guy at the shop was very knowledgeable and had all the right tools, but I did not notice that the picture @thefreeman originally posted has the “+” and “R” pins reversed in relation to the ones in my adapter 
so initially he did the mod to the wrong pin (the “R” one) instead of the correct “+” pin)…
Also, when he cut the copper trace connecting the “+” pin to the rest of the circuit, the onboard PWR LED would no longer light up… turned out that the LED is powered from that copper trace too, so if you don’t want it to go off permanently, you have to cut the trace very close to the “+” pin.
Fortunately the cellphone repair guy was not only knowledgeable but also very patient, and so he undid the mod to the wrong pin, then did it to the right pin, and then (when the PWR LED refused to turn on) traced the board with a DMM and diagnosed and fixed the issue (by reconnecting the trace where he first cut it, and then cutting it again at the point I indicate below).
For easy reference, here’s @TheFreeman’s picture, along with a photo from my adapter, depicting both issues:
I hope this helps other people avoid these pitfalls… 
EDIT: changed the picture to try and make it clear that in the end what worked was the wire jumper mod alone – no capacitors in my final setup.