Instead of trying to calculate extra attacks, what about having BABs offer an attack speed bonus instead?
Say:
BAB 5 (+5%)
BAB 10 (+10%)
BAB 15 (+15%)
BAB 20 (+20%)
Adjust numbers as you see fit. Since we have the mechanism in place already to do this (Tempest, Zeal) in theory it should be somewhat easier to implement. Just make these inherent bonuses so they stack with everything else.
Just a thought.