I too had seen this (as well as had informed about it to Marek over irc) but later couldn't dint spend time replicating it.
For some reason batman was not working with INVALID state detection in FORWARD chain.

So Philippe is your problem solved ? i am in the process of upgrading the firmware to latest openwrt trunk. as after rearranging nodes and fixing diversity my hopping problem is some what solved but batman on gateway still crashes after around 6-7 hrs of uptime on Kamikaze 7.09 (kernel 2.6.21)