[noggin]

Could connect to a USB 3.0 hub & Ethernet adapter combo. I use this one w/FTV3 pendant w/good results.

 Don't quote me on this, but I want to say that the USB port on the Fire TV devices is 2.0, so you're not even getting half-gigabit speeds.  

Yep - same as a couple of platforms that need USB Gigabit connectivity to replace their built-in WiFi for UHD rips and only have USB 2.0 connectivity. You don't need full GigE for these.

Granted it will be a lot better than wireless and the 10/100 adapter Amazon sells, but I'm curious how you plan to get around there only being a microUSB port on the FireTV, which is used for the AC power feed.

Taken from the product specs: Ports HDMI output, micro USB for power only 

  
Amazon and Google both have PSUs, or power pass-through, with integrated Ethernet that use OTG + Power over a single connector. You can get third party adaptors that also allow OTG + Power. (I've done this with a Fire TV 4K Pendant)

You can also buy third party OTG hubs that do this, which would allow a USB 3 (over USB 2) GigE adaptor to be used.  As long as drivers are present (which for some chipsets they are) then you are good to go.