I finally managed to get AGS to start using the instructions above, but as soon as I try to move around in the AGS menu, it returns back to the A600GS main menu. The same thing happens with the keyboard, a USB gamepad, and an Arcade R 9-pin joystick.
EDIT Actually, on the very first launch it seemed to work. I was able to move around in the menu and tried one game, and it worked normally. I shut AGS down, and a little later when I started it again, this issue began: moving in the menu sends it back to the GS startup screen.