El problema

Al ejecutar:
ebtables -t broute -A BROUTING -i $CLIENT_IFACE -p ipv6 --ip-proto tcp --ip-dport 80 -j redirect --redirect-target DROP
Devuelve el error:
for ip filtering the protocol must be specified as IPv4

La solución

ebtables -t broute -A BROUTING -i $CLIENT_IFACE -p ipv6 --ip6-proto tcp --ip6-dport 80 -j redirect --redirect-target DROP