not abscess--not navicular either--UPDATE
Xrays showed the hole up her hoof wall was very deep; the vet said the mare's soles were very, very thin. The wetness, lack of hoof support, probably the cause of bruising---there was evidence of some deterioration to the front of the coffin bone. (He had a box of horse-foot parts so he could show exactly what he was talking about.)
Also showing up: old fractures; the start of ringbone; irregularities here and there. But the navicular bone looked okay--for now anyway.
And after all was said and done, there's still this tiny doubt that the "bad things" are causing the pain. Or that what looks okay really is. Oh well!
The plan is to have my farrier (she'll be consulting with the vet) cut away the hoof to the "living hoof" and cast it, encouraging the growth down to adhere properly. And padded shoes. I asked if that wouldn't make her soles even thinner, or more tender; but the vet felt that, in this wet area, they might make them stronger, but she had to be kept as dry as possible. It's raining as I write. The vet also didn't like giving her so much bute. None, if possible. So my little horse is kind of sore right now, but having her 2-hours grazing (in the rain) and happy for now.
I hope those of you who, like me, hesitate at calling the vet, go for the x-rays at the first go-round. Would have saved me a couple hundred dollars, and a lot of guesswork.