I got IPv6 working with the following changes:
In lpc 18xx_43xx_emac.c
add LPC_ETHERNET->MAC_FRAME_FILTER |= MAC_FF_PM;
You need to be able to receive mulitcast packages for ipv6 Neighbor Solicitation.
and
add add this case "case ETHTYPE_IPV6:" to the switch in lpc_enetif_input.