Do you need it to fix the "mouse pointer lagging" problem with fglrx? This patch might be what you need. It works for me with Half-Life and Jedi Academy.
I kinda see how this could help, but it would need to be better understood first before being applied (it would need, of course, to not link directly to GL and use function pointers :-) ).
Heck, it should make performance almost worse as we add a round-trip to the X server to do this and need to flush the GL rendering pipeline at each buffer swap instead of continuing sending commands to display the new frame while the swap occurs...
Lionel