A few have done things.
Most just tap the headphone port and fly a speaker to space wherever. Some might stick an extra amp in there too.
Don't know offhand what exists on the board itself if tapping the headphone port is not your method of choice.
PCB shots if someone fancies having a look
https://bitbuilt.net/forums/index.php?threads/gba-board-scans.807/
It's been done before. Can't post the link to it cuz I'm still a new member, but I can come back to post it lol.
The guy used Nintendo Switch speakers and the PAM8403 stereo amp board. Looks like he didn't wire a shutdown for the amp though so the speaker would still run with headphones plugged in.
I know if you wire A- of the PAM8302A mono amp to pin 5 of the headphone jack it effectively acts like a shutdown for headphones (don't know the exact details why that works), so maybe something similar could be done here?
You absolute star!! Thanks so much, that's a shame you can't shutdownI finally got regular member privileges!
Anyways, here's the link to the mod I was talking about.
This thread here also is where people went into the mechanics of wiring a shutdown for the amp boards. It seems there's no way to actually shutdown the boards when headphone are plugged in, but it gets it silent enough that the extra power draw is negligible.