Yes, I understand your concern. But I actually accidentally had this type of situation at our old place. We put a round pen in the corner of our field. There was a gradual narrowing where the round pen met the square corner. (You need to visualize this as it's hard to describe.) And there was a little gap between the rounded edges and the hard square, so our little foal was able to get back in that corner all by herself. Mom would go in as far as she could, but she couldn't access that back corner. Viola! Instant creep feeder. I just put hay and foal pellets back in that corner, and little Anna came and went as she pleased. Mom was prevented from going in any further due to her size, so she backed out. (Which, incidentally, this mare backs up just fine and does not have arthritis as previously suggested by someone.) If I'm describing this adequately for you to visualize, the joining of the round pen in a square corner created a "chute." No other horses could enter. Mom's butt was in their faces. No accidents, no panics, no runs, no drips, no errors. I sure wish we were still there. Of course, if we were, we'd have 3 separate pastures, two loafing sheds, and the barn that someone suggested I find more time to be in!
I'll find the perfect solution for my old girl. I've had her since she was two, and the others were born into my arms. I'm going to win this one! Although I suspect they're already conspiring to foil my plan!