The fly mask was really a non-event!
I stalked Mezza, and worked with her, doing in-hand and longing before introducing the small fly mask that a friend had used for awhile on her horse. I let her feel the soft edges, and mouth it a little, then I proceeded to rub it on her head, neck and over her ears before finally taking it off and on several times.
We finished with me leading her around in the round pen with it on, but taking it off before taking her back to her own pen.
Then, several days later, I again put a fly mask on her in the round pen, and practiced leading her in it at the walk, then the trot. I took it off to walk her back to her pen, but put it on again once she was back in her pen, and left it on her for about 10-15 minutes. She did not seem flustered.