FAR 25 Amdt 92
Since the above amendment was issued it has been permissible to consider the use of reverse thrust for the accel-stop case on a wet runway for a FAR 25 certified aircraft. I suspect that the 747 graphs referred to in the original post have been compiled in accordance with this latest revision to this part of the rules.
I think the current JAR 25 regs are similar, but can't check them here.
Therefore I'm afraid that the definitive statements being made to the effect that reverse thrust is not a cert issue are not strictly correct.