On a few stripped nuts I have been able to force the nut outwards while turning, that may be hard to do on the head without hurting anything. Think possibly heating it up could help? Maybe call a BMW shop or a machine shop familiar with BMW engines and ask for advice. As a last resort you could get it with a dremel, just grind the nut off and replace the bolt and nut. Make absolutely sure you use enough rags or bags or whatever to keep the particles out.(I'm picturing the little rubber dam thing the dentist uses) This sounds risky to me, hopefully someone else will have a better idea.
PS maybe repost it with more info in the subject line, I almost didn't read it because there is so much already about valve adjustments
Good luck with it,
Tom