Why limit it to BAB? By the time a fighter qualifies for the WWA/GC combo (6th level for human fighter and 8th level fighter otherwise - assuming they don't go the weapon specialisation route along the way), then they'll pretty much be at their maximum use for Great Cleave anyway if they are surrounded on all sides (exluding 3D situations) by small creatures or larger.
If a 6th level fighter is willing to place himself in that situation, then either he is overly confident in his abilities, an incompetent tactician, or desperate.
Just strike dead any character using the bucket of snails or similar cheese (doesn't 3.5 have rules for swarms? That might be a better fix).
Let the Great Cleave hit any creature, not necessarily the next one in the circle, and let the poor fighters thrive - after all, feats are the only thing that they have available to them to separate them from the hordes of characters with special abilities.