I screwed up the contacts that TRIAC pointed out, but I soldered instead to the ROUT and LOUT just above the jack and am using a 3.5mm jack I cut off a shit pair of earbuds to force stereo. I got a pretty clean signal off of that and I'm happy with the result
https://soundcloud.com/noyzbot/punch-on-rca-mod
I'll update with pictures inside while I put the mod in my afterburner GBA so it's more complete, but there are other pictures in this thread that work just fine to illustrate.