/usr/share/<greeter name here>
But default gtk greeter isn’t big on customization, you need webkit for really fancy stuff.
Openbox’s autostart only works after you login. If you add a command to
/etc/X11/xinit/xinitrc it will start with X-server (that’s when the display manager starts). Try compton command with necessary options in xinitrc, but like i said it may not work. A compositor needs some window manager (openbox, i3wm, kwin, marco etc.) to work. Not sure if greeter interface provides such a facility. I cannot test this at present, using different greeter here