OK. Answering my own post here. I set up an standard http proxy to the origin server and it seems to be relaying HLS as expected.

So the rtmp application is now...
application live {
live on;
pull rtmp://89057213.r.cdn77.net/89057213;

}

And the server config is now...
location /live {
proxy_pass http://89057213.r.cdn77.net/89057213/;
}