I watch a lot of anime, tend a few conventions annually, and play lots of video games, and not a thing comes to mind when I see bagel head. Def not a cosplay thing. :lol: I think it's just seeing how far they can take body modification. People used to cringe at gauged piercings, facial piercings, ect, but now it's so widely accepted and common, hardcore body modifiers are probably just trying to see what else they can do that's unique and out of the ordinary. It's seen all the time at tattoo and piercings conventions. People poke holes in the weirdest places, expand piercings that normal people wouldn't, and implant all types of stuff in them. (There's a guy that has magnets implanted on his wrist so it could hold his ipod nano without a watch strap!).
That's just my speculation though. No matter what the reason is... it's definitely weird!