non-blocking SwapBuffers() with VSync=on

I am looking for a portable way to make a non-blocking SwapBuffers() even if VSync is activated.
In other words, is it possible to to be notified by an event or to know the delay until the next VSync ?

What does swapping buffers mean?

A swap chain is a collection of buffers that are used for displaying frames to the user. Each time an application presents a new frame for display, the first buffer in the swap chain takes the place of the displayed buffer. This process is called swapping or flipping.

What is swap interval?

The term "swap interval" itself refers to the number of v-blanks that must occur before the front and back frame buffers are swapped. A swap interval of 1 tells the GPU to wait for one v-blank before swapping the front and back buffers.

IIRC this extension helps: http://www.opengl.org/registry/specs/SGI/video_sync.txt, but it is very poorly supported with current drivers.