Thanks for the input!
So it's worse this morning (she still doesn't seem to notice) but also looks more like smaller darker bruises in addition to the diffuse/fading ones in the pic. I'm starting to wonder if one of her sisters is going after her feet at night, as the size of the new dots looks suspiciously like how my arms look when I get nibbled too hard. The first time I noticed this some weeks ago, two geese had it, though this one much worse, and the one who likes to nibble her sisters had nothing. Which is suspicious! I may need to put a camera in there and spy on them at night. At least the bruising doesn't seem to bother her at all? These baaaabies.
They have access at all times to a shed with deep pine shaving bedding. When it's really cold they do go in there voluntarily during the day and then get shut in for the night as soon as it's dark. It's supposed to get cold cold this weekend though so they may be staying shut in for the worst part of the day at least. Though if the problem turns out to be boredom nibbling then that'll probably just make it worse. Do they have toys in there? Yes. Do they chew on each other anyway? Also yes.