UPDATE: See below main post.
Alas, one of the girls has just developed severe lameness. She's in so much pain she doesn't want to stand at all.
When I picked her up she feels a bit too heavy for my liking -- while it's good that the home-mix diet is allowing them to develop a lot of muscle (she's not fat, just very solid), the huge weight will make recovery hard.
There does seem to be a lump on the foot pad, and I feel she may have cut it on something. The former owner of this property used that part of the yard as a rubbish tip, and it's possible the girls have scratched up some glass or sharp metal. After checking her foot more closely I'll examine the yard.
At the moment I've got her in a cage on a soft mat. When she's feeling a bit more settled I'll wash the foot and have a closer look. Bumblefoot is treatable but to be honest in such a heavy bird it may be kinder to cull. Chickens this heavy simply can't spend a lot of time on one leg, and often when there's an injury, the other joints are put under such stress they get damaged as well.
But I'll take a closer look later today, and decide what to do. It may be a simple cut that just needs cleaning and some time on soft clean matting. I'll also limit the girls' food a little more so they're not putting on quite so much weight. In a few weeks' time I'll be able to open the doorway into the rooster pen, and then all the birds -- including the girls -- will get a little more exercise.
Unfortunately this isn't as simple as a cut foot. Her foot is perfectly fine (in fact it's the non-limping foot that has a small scar). Neither foot is red or swollen.
It appears the lameness is in the hock or higher. These birds were all vaccinated for Marek's, but I'm aware of arguments that vaccinated birds can shed the live virus, and can also develop symptoms. Given the fast onset, and her huge weight, I feel it's much more likely to be a tendon problem due to weight-stress. This is a big problem in meat hybrids kept to adulthood. She may even have hurt herself getting down off the 5-inch-high roost.
Whatever the case, I feel it may be best to put this bird down (checking for any obvious internal problems — something I do out of habit) and concentrate on limiting the diets of the remaining 6. It's a shame as she was quite close to point of lay and her comb was reddening nicely. On the other hand she's gone along for nearly 3 times the lifespan of supermarket meat birds, and I know she's been cared and had a low-stress life till now.