One solution is to share the same IPv4/IPv6 and also MAC address, to avoid ARP timeouts

ip link add link br-lan anygw address aa:aa:aa:40:28:b5 type macvlan
ip address add dev anygw fddf:ca40:28b5::1/64
ip address add dev anygw
ip link set anygw up
ebtables -A FORWARD -d aa:aa:aa:40:28:b5 -j DROP
ebtables -t nat -A POSTROUTING -s aa:aa:aa:40:28:b5 -o bat0 -j DROP

this solution works only when every node is doing layer3 dynamic routing (i.e. BMX6 everywhere, anygw interface everywhere)

Another possibility is not to share the same MAC address, instead relying on ARP timeouts, and the fact that batman will forward packets. The downside is suboptimal routing when roaming.

Third possibility is having batman correctly support anycast MACs; say "batctl anycast_prefix aa:aa:aa:00:00:00/24" that will make batman allow those MACs to be claimed by many nodes (preventing roaming of those MACs).

These last two options allow not running layer3 daemons on every node, only on the borders