Well, here is a much belated update for anyone interested.
Originally, I had replaced the charging port on this Shield Portable with a new one from eBay. This worked to charge the device for a month or so, and then the resoldered port broke off. There were several pads lifted after the break, when I originally posted this thread, but 5V and ground were still intact. This is what the pads looked like:
My unsuccessful attempt at fixing this involved soldering a red wire to the 5V pin and a black wire to a nearby ground pad. I soldered the other wire ends to a PCB breakout board after drilling a hole through the back of the shell casing. I didn't have Mode Detect, D-, or D+, so those were ignored. Also, the only bypass points for Mode Detect, D-, and D+ that I found on my spare board were too small to solder to on miniature IC pins near the charge port.
Here is 5V and ground soldered, which I later covered with white silicone rubber adhesive and let dry:
I found it was too difficult to solder another tiny capacitor from my spare board to the one missing on the 5V rail, so I left that part out.
Here is what my project looked like after soldering the wires to the breakout board and gluing it down:
I plugged in a good 5V 2A micro USB charger into the new breakout board port and let the unit sit for a few hours in sleep mode at 75% charge. I noticed there was no charge to the battery pack after a few hours of letting it sit (battery life actually decreased to 74%). I was thinking perhaps the red 5V wire came loose on the motherboard and peeled off bits of the white silicone cured adhesve to get to the 5V pin.
Unfortunately, this and other messing about at the hinge caused the delicate screen ribbon cable going to the LCD and digitizer to break. It was unrepairable for me after that. I looked and there were no spare LCDs available for this older 2013 Shield Portable model on eBay. A used Shield runs about $160.
In retrospect, had the bypass PCB board worked to charge the device--which I think in theory it would have--I would have still been concerned about possible fires or other issues if the console was left unattended. If the 5V pin was bypassed elsewhere on the board, or directly somehow to the battery (dodgy), I would have also been concerned.
I have since disposed of this project and the console entirely. I got a good 7 years or so out of it, so there's that!