I've just tried the same with my A600GS. I get a "Software Failure 8000 0003" after choosing the Moon Patrol AGA slave option. Firmware version 46.2.1069.
Yes, this is exactly what I am getting now.
What makes is even stranger is that in past firmwares it was working perfectly, but now using 46.2.1069 it won't work.
I have the correct kickstart files, and have not changed them since I originally installed them when I first got my A600 last year.
And this is exactly what is happening with any WHDLoad game I try to add that isn't natively supprted by JST.