But you're still limited to one Cleave attempt per round, correct? Even if you have Combat Reflexes and a 37 Dex, no matter how many provoking dire lemmings you swat, you can only hit your actual opponent one extra time per round.
The only time it would really offend me would be if the conjurer in the group routinely summoned CR 1 monsters to run circles around the cleaving fighter every round so that he would always get an extra attack on whatever opponent he was facing (though, if the opponent had cleave, he would likely get one as well, unless the lemmings ran back and forth behind the pc). Behavior like that would get the party a visit from the CR 30 God of Dire Lemmings (unless I was running a game designed to reward players for taking advantage of loopholes, which might be fun too.)
But shy of that, in the course of normal combat, one cleave attempt per round doesn't offend me, whether it comes from normal killing, or opportunistic killing. That's what cleave does.