As long as you use some sort of gl output it seems like it should be synced like any other directly rendered surfaces.
The problem is, there are no standalone small gl compositors (standalone = just compositor, not a combined compositor/window manager). I wouldn't want to give up on the feature-set and functionality of openbox. There's dcompmgr, but it hasn't seen development in years, and last time I tried it, its opengl mode caused a lot more CPU usage when doing simple things like moving windows around the screen.
Now I don't have tearing issues with my current hardware, but no idea how it would be if I had Sandy Bridge or above.
psh. i'll be impressed when i see it run a program that syncs itself to vblank, running in a window without it either having to drop or buffer frames from said program. seriously, fuck this culture of excessive buffering of frames! IT"S NOT OK! as john carmack recently pointed out, it's faster to send a packet across the globe than it is to get a pushed frame displayed on your monitor nowadays. (yes i know it's primarily a fault with displays, but software should do whatever it can to not add to the problem)