I was thinking about it today when I brought her inside and think that might be what it is. Her mother and father are vaccinated, but since she was hatched under a hen she is not. Considering that all the birds I've lost to this mystery illness were NOT vaccinated (6 birds now in 2 years) and all the vaccinated ones haven't...the puzzle pieces are just fitting together. The only weird thing is I sent one of them in for a necropsy (he was a silkie who went lame, got skinny, AND had wry neck) and they said no to Mareks...and said he had hepatitis or something. I had three go lame and lose weight and three who just got skinny and died. But why wouldn't it affect ALL the unvaccinated birds I have? Her siblings seem fine and the unvaccinated banties are fine as well. I lost one in that coop to it but have three more who appear unaffected.
I think that's what she has, unfortunately, and I'm debating whether I should put her down as soon as possible, let her die naturally, or put her down when she stops eating or something.
*sigh* I guess no more broody-hatched chicks for me :\