I know it's kind of too late ask, but all information I could find was a really old reddit post referring to Mi Box 3 Marshmallow.
And I know it's working well with Shield TV and Bravia as voice-input device, but how about with Mi Box S Oreo(or Pie) ?
Didn't even think of the possibility, but I just hooked my old PS3 Eye to my MiBox S, and it works!
You still have to press the Mic button though, no "OK, Google" recognition, unless there is a setting for that somewhere.
The mic works pretty well, though - at 12ft distance, it had no problem catching "Play Tensei Shitara Slime Datta Ken, episode 1" on the first try, despite my Japanese pronunciation definitely still needing some work.
Photo of TV with PS3 Eye
A little disappointed it didn't pick up results from Plex, but that's a different issue.