Any instances of him signing throughout his tour? I know he is tough and I've already had a less then favorable experience with him, but do you think he's more likely to sign one of his solo albums? He's quite curmudgeonly.
Probably time to buy an authentic Plant before prices go up. A guy that sells Rock stuff told me he came into one of his shows to buy a Photo of him with a dove on stage that he hadn't seen. He wouldn't sign for him but eventually did when he traded a signature for the framed photo.