The bird was indeed cooperative, staying relatively close given that it has acres of airfield to roam about in, though wasn't close enough for the 300mm lens to accomplish much. I had to revert to digiscoping at 60x to get anything workable, with my wife's camera since my old A620 wore out earlier this spring and I haven't gotten around to replacing it. I'm definitely out of practice.
The bird's proportions are off a bit, with a coved-in area at the base of the neck and upper breast. At first I thought I was seeing the crop bulging (as it was actively feeding), but soon realized that it was the breastbone bulging out, a conclusion that others had also come to.
Here's another shot of the breast bone.
Here's one from California in August of last year for comparison.
Time will tell if this is a sick bird that is falling behind calorically, or if it just needs to refuel for the rest of migration. My understanding is that Eskimo and Bristle-thighed Curlew would/do lose about half their body weight in trans-oceanic flights, but I doubt that's the typical pattern for Long-billed. It was certainly actively feeding so hopefully it will fly past Tiscornia in a week or so.