It's definitely not an impacted crop. I've done the treatment for that, just in case, but it hasn't helped at all. Plus the lump doesn't change size through the day, is quite soft and it's very mobile. I can move it from its natural starting point (just in front and below the right wing joint) to all the way under the wing, and back all the way in the middle of the breast area, without the chick acting like that's uncomfortable at all. I don't think the crop would move that much, or at least, it'd hurt.
I've been doing research online and the best guess I can come up with is a fatty tumor. I can't afford to have it removed (even if there was a vet around here who'd do it, which is doubtful), and bleeding's supposed to be a big problem in birds, so I guess I'll just have to let Lumpy keep his/her lump. It's not bothering Lumpy at all--still eating, drinking, and pooping as normal, running around with full energy, etc. Too bad I won't be able to breed him/her, because s/he has one of the best, fullest tufts of the whole crew.
(edited to add)
They do get treats--a crumbled boiled egg yolk between the 10 of them every day or two--and have gotten a bit of grit, too, sprinkled over the top of their crumbles every other day. Mostly, however, they're just on medicated chick starter.