Synchronous Timewarp!
(not to be confused with Asynchronous Timewarp)
Render extra timewarp frames per frame if needed.
Set Min extra frames and Max extra frames.
It should work for anything with Min=0, Max=89. But just in case it doesn't:
Use Min=1, Max=2 for 30 FPS games (Wind Waker) at 75 FPS. Use Min=0, Max=1 for 50 or 60 FPS games. Use Min=2, Max=2 for 25 FPS games (PAL half speed). Use Min=0, Max=0 to disable.
Known issues:
This is hardcoded for 75 Hz, I don't know how to detect the Rift's current refresh rate.
There seems to be a bug in the VR FPS counter, sometimes it registers lower than it really is (single digits), I can't work out why.
Setting it higher than your computer can handle will slow the game down.
I don't know if it works at 75 FPS or it drops frames, because my computer can't run that fast.