Well, I got good news and bad news.
Good news: I've found out how to install the .trx image: just configure your computer to a fixed IP address in the 192.168.1.x range (x between 2 and 255), reset the router while keeping pressed the reset switch on the back for 20 seconds, and access the CFE mini web server at 192.168.1.1. There you can upload the LEDE .trx image. 
Bad news: LEDE only supports the 2.4 GHz Wi-Fi, and only in B/G mode, not even N. So, it sends your router back to 2005 
So... you can revert to the factory firmware on the CFE mini web server. 