This probably applies to anyone, but for those with the Mi Box who want more functions in your Harmony remote, you may want to try to email them.
If you want, try e-mailing service.us@xiaomi.com asking for their Hex/Pronto codes and/or for them to send them to Logitech at datasupport@logitech.com .
Logitech will only by default replicate the buttons on the original remote. However, once received, Logitech will add all the codes that Xiaomi uses. Xiaomi has the standard Android TV OS controls in their IR codes but doesn't use them for their remote.
I doubt I will receive a reply for my email I just sent, but maybe if enough ask they will take 2 seconds to hit the send button to Logitech.
I really don't understand why Logitech doesn't use bluetooth for the Mi Box like the Nexus Player. For me the only missing buttons are play/pause/FF/RW. But I mapped those to my TVs functions and it works great over hdmi-cec.