Our grass may have had something to do with it. We've had unusual weather this spring -record-breaking amounts of rain, causing rapid growth of the grass, followed by some hotter, drier weather that may have stressed the grass. We've got some seedheads already, and we usually don't start to see those, and do our regular mowings, until later in June. I expect the fructan levels may be higher than normal. Since Maisie's a little overweight anyway, which isn't good for her feet and joints and may predispose her to laminitis, I may just keep her on dry lot through June, and then take her back out to pasture in July with a grazing muzzle. Since Lily is getting fat as well, I may bring her in to the dry lot in the PM during June to be with Maisie.
Update at around lunchtime: I just got off the phone with the vet, and we have very good news. Her feet, except for a little arthritis, look very good - no rotation of the coffin bones at all. The only other notable thing is that she has unusually thin soles, and will need to get her shoes back on once she's more comfortable. Starting tomorrow, she can go out in the dry lot with Lily (mud lot would be a better description!), as long as she doesn't run. I think she'll stay calm as long as Lily is there. She'll stay on her meds, tailing down the Banamine starting Saturday and staying on a low dose of Bute until she gets her shoes back on. The vet will come back on Tuesday for a check up.
It seems as though we may have caught the laminitis just as it was getting started and before any serious damage was done. Assuming she continues to improve and stays comfortable, and is able to get her shoes back on next week, we may be able to start work again pretty soon. I'm feeling mightily relieved - this is my first (and I hope last) brush with laminitis.