I would be inclined to try ivermectin because it may be depluming mites which are microscopic. I too do not believe it is Marek's. Usually there are lesions with Marek's and all I see is dry irritated skin. Is she a frizzle? I know some of those varieties and crosses have poop feather quality that easily break and she doesn't look totally smooth feathered although it is difficult to assess with her plumage being so damaged. Broodies are at greater risk of mite infestation because they are confined to the coop for long periods with very limited time and perhaps no facilities for dust bathing, so I would be leaning towards parasites causing this as since it occurred whilst/after she was broody. I believe a vet could examine skin scrapings/feather shafts for the microscopic depluming mites. Probably better to do that than reach straight for the ivermectin, since it is not registered for use in chickens and you would have to figure out a withdrawal period for eggs that you were comfortable with.
