Unfortunately, I do not have a guess as to why your curl call does not yield you any data. However, I wanted to share a similar observation I've made with regards to the Yun not showing up via IP in the Ports menu of the Arduino IDE. I've noticed that when my Yun is connected to my computer with a USB cable only the COMX port is available from the IDE. However, when I look for the Yun from a second computer connected to the same network as the Yun, but not physically via USB, then the Yun does appear in the Ports menu with an IP address. Do you have a second computer you could try to identify the Yun with?