I doubt it would have made that much difference. From what I've read, most of the people killed were either above the initial point of impact or were rescue workers who were not, at the time of collapse, trying to leave the building.
When trying to fireproof a structure, there's a tricky design issue of whether it's better to have the insulation over the structure as a whole (in which case heat from a fire in one part of the structure can dissipate into other parts, preventing localized meltdown), or whether it's better to have individual parts insulated separetly (in which case heating effects will be localized, but there's more likely to be a localized failure).
I don't know which design approach was used in the WTC, but both would have been problematical. My guess would be from the pictures I've seen that the former approach was used. Such an approach would be best in case of a fire which was not accompanied by much structural damage (most fires would probably fall into this category). In such a case, all that is necessary for the structure to survive is that the metal-to-metal heat tranfer away from the fire through the ends of the involved beams be fast enough compared to the air-through-insulation-to-metal heat transfer from the fire. Given that the frame of the building as a whole has a very large thermal mass, this would normally not be a problem.
The plane crash, however, created a problem: many structural members were ripped apart. The loss of structural support from such members would not have been a problem, but unfortunately the ends of such members would have had substantial areas of exposed metal. While heat damage to these elements themselves would not have been a problem (they were structurally useless at that point anyway) they provided a means by which heat could be conducted to the elements that had up to that point not failed but which now had to bear substantially greater than design loads.
BTW, I forget the exact figures, but I calculated the amount of potential energy unleashed with the collapse of that building. That was a pretty darned big number.
According to the NOVA story tonight, replete with good animations, the blast of the impacting jets blew away the foam substitute. It's unclear to me whether asbestos would have been as easily blown away. Maybe it would have, in which case the point's moot.
Well, I don't know how a building could be redesigned to be terrorist-proofed against hijacked aircraft, but perhaps instead of the building design being the issue, maybe tall structures should be armed with surface to air missiles that could take out a 767 or 747 on a head-on impact course.