Yes, that’s a very good point. Thanks! Whenever I’m exchanging the bridle for the grazing halter, for instance, I put a leadrope around her neck and hold on, just to be safe.
BM has said she’s willing to stand with her with the lead around her neck, and I was tryinig to make it even less of a nuisance than that. But you’re right, trying to put some kind of a blocker into the grazing muzzle is even more awkward and time-consuming. 
Swapping with an actual feedbag would make more sense.
(At the moment I believe BM has tried the treat method I use - drop a treat into the grazing muzzle before putting it back on. She says as of now mare is waiting for her grazing muzzle after finishing her supplement. 
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