Phoronix: Tear-Free, Hardware-Accelerated Video On Wayland
Kristian Høgsberg has demonstrated that tear-free video playback on Wayland is a reality and that running Wayland on Intel hardware does allow for hardware-accelerated video playback thanks to VA-API support...
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.