How-to include IP address(es) of some balanced Web Servers into HTTP Header?

You can create a data group with all servers for which you want to insert this information.

Then when your data group is created, your iRule will be :

when HTTP_RESPONSE {
if { [class match [IP::server_addr] equals my_ip_datagroup] } {
    HTTP::header insert Server_IP "[IP::server_addr]:[TCP::server_port]"
}