This happens at my barn all the time.
Boarders decide to only come a few times a year, and leave their horses to us. We'll take care of them, but we don't love on 'em, groom them, and all that.
Right now, there's a Rocky Mountain horse gelding named Mountain (lol) who is very aggressive towards other horses and will pin his ears at people. He hasn't tried to bite anyone yet though. My manager hates him because he picks on her gelding, but he's just grumpy.
So to give him something to do, instead of just pouring his grain in his bucket, I feed most of it by hand. I take a handful and make him stretch for it. To his flanks, between his knees, anything I can come up with. I've only been doing it for a few days, but I already noticed that he doesn't pin his ears at me when it's time for a meal.
I figure since I'm not feeding him any treats, only the grain that he'll get anyway, it's not that big of a deal. I'm not really worried about his owner walking in me because out of the 2/3 years that Mountain has been at our barn I've never seen his owner.