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."
"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."
[1] https://docs.aws.amazon.com/elasticloadbalancing/latest/netw...