Just tested a USB ethernet dongle for my Mi Box. I can confirm this is working. Sideloaded speedtest app and on my work internet (which is gigabit), I did a speed test and had no issues. Granted it didn't get full gig speeds due to the USB 2.0 port but it was almost maxed out for 2.0 speeds.
I used a combination of these two:
https://www.amazon.com/PUREX-Technology-Dedicated-USB3-0-USB2-0/dp/B00D8JZ05A
https://www.amazon.com/TRENDnet-Ethernet-Chromebook-Specific-TU2-ET100/dp/B00007IFED/
I bought the USB hub at Microcenter for $10. Amazon's price is crazy high.
I know the port on the Mi Box is 2.0, but I plugged a USB 3.0 64GB stick in the 3.0 port and then the trendnet ethernet adapter in the 2.0 port. Using the 64GB stick for internal storage which is working great. Does not need external power.
Also for a test I tried an Apple USB Ethernet dongle which also worked. It's not hot swappable though. After plugging it in you have to restart the Mi Box.