So I got around to trying to build from zyxels sources and it just worked, the issues I ran into compiling from the github repo disappeared.
Just follow the instructions that come in the source package and out pops the image.
Troubleshooting the previous build did reveal something interesting though, one of the things that failed to build initially was conntrack-tools, and watching the boot log with a serial terminal it seems even in the official firmware its not built correctly:
Netfilter messages via NETLINK v0.30.
nf_conntrack_netlink: Unknown symbol __nf_ct_l4proto_find
nf_conntrack_netlink: Unknown symbol nf_conntrack_find_get
nf_conntrack_netlink: Unknown symbol nf_conntrack_register_notifier
...
nf_conntrack_netlink: Unknown symbol nf_conntrack_l3proto_generic
nf_conntrack_netlink: Unknown symbol nf_ct_unexpect_related
nf_conntrack_netlink: Unknown symbol nf_ct_remove_expectations
insmod: can't insert '/lib/modules/2.6.30/kernel/net/netfilter/nf_conntrack_netlink.ko': unknown symbol in module, or unknown parameter
Not sure what effect not having this module working has if any, conntrack-tools appear to be for monitoring stateful packet inspection.