I haven't been able to make an ELB target be an external IP. What did you mean by "ELB's application load balancer seems to able to point to AWS external IPs"?
IP addresses as Targets
You can load balance any application hosted in AWS or on-premises using IP addresses of the application backends as targets. This allows load balancing to an application backend hosted on any IP address and any interface on an instance. You can also use IP addresses as targets to load balance applications hosted in on-premises locations (over a Direct Connect or VPN connection), peered VPCs and EC2-Classic (using ClassicLink). The ability to load balance across AWS and on-prem resources helps you migrate-to-cloud, burst-to-cloud or failover-to-cloud.
Looks like you need an active VPN connection to access external IPs.
That feature requires you to use a private IP address, so if you have a VPN or Direct Connect to another location you could load balance across locations. In the case of the global load balancers those will be public addresses though.
"The IP addresses that you register must be from the subnets of the VPC for the target group, the RFC 1918 range (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16), and the RFC 6598 range (100.64.0.0/10). You cannot register publicly routable IP addresses."
