As one who also suffers from the same ailment on occasion, unfortunately, I don't know any way to set the aspect ratio of thumbnails on the sets.
Having a touch of OCD, I like to get everything as correct as possible on my media server. One thing I cant figure out is the actual size of the thumbnail images in the Media Play browser. Does anyone know? I know the TV will resize them if they're not right, I'd just prefer them without the rendered jaggies if possible.
I use Mezzmo (the best media server I've tried so far) in case that makes a difference.