One addition:

Actually, I am not trying to proxy one server (as I learned, this can be done by just assigning "proxy_set_header Host " someServer.akamai.com", but if possible several (I just reverted to one here for boiling down the issue).

So I am using an upstream directive like this
upstream akamaiBackend {
server someServer1.akamai.com:80;
server someServer2.akamai.com:80;
server someServer3.akamai.com:80;
}

Maybe it is possible to define the header_host to be forwarded in the upstream directive? Haven't found it in the docs, though...

Thanks
fatzopilot