Jamie is one of the only players who I have ever seen sign a nickname, his is "Cat", in their autograph. Every autograph I have seen of his includes Cat, so I guess it's something he does all the time and it makes his signature pretty unique and different.
In 1994, he played as a replacement player in Spring Training during the strike, so he was excluded from being a member of the Players Association. That, combined with the fact that he was a middle reliever, means that he was only featured on two cards during his ten season career, both as a member of the Tigers in Topps Total sets, so I was pretty limited with what he could sign. Jamie signed the Heritage team card for me in person at an O's game in Washington during the '09 season, and I bought the signed team postcard online recently. Jim at GCRL made me a few custom Jamie Walker Orioles cards last year and Jamie is(or at least was) a good TTM signer, so I might send one of those out to him one day.