just use a kama with the vicious stomp feat and combat reflexes and trip everyone as part of your flurry for free attacks of opportunity on top of that. what the problem is?
jk. on a more relevant note I don't think its that unbalanced really. there are similar feats that let you use other weapons for flurry that just have more specific requirements or limited selection. You could probably find a way to make it broken with those feats, but if your intent is more just for the flavor of flurrying with your weapon of choice I think that'd be fair for at least a one time exception subject to change at the dms discretion, if the dm is ok with it.