BASE jumpers pack their 'chutes to open more quickly than jumpers at altitude do.
IMHO (and I have done some skydiving), there would be plenty of time for a chute to open, particularly if it were packed for a fast opening, when falling from the top floors of the world trade center. As an example, one BASE jumper has video of his jump from the Empire State Building observation deck. His chute opened with AMPLE time to spare.
However, the falling debris would likely have ripped the 'chute apart, or caught the 'chute on fire, and as others hypothesized, multiple jumpers would likely have become entangled.
This is, of course, a purely academic debate.