I haven't heard of the connection to Mad Magazine, however I know that Creepshow's producer Richard P. Rubinstein grew up next door to Jack Kamen, and his parents were good friends with him. It was a reason why Kamen was commissioned to do the poster art for Creepshow.
I'm actually working on an audio doco about the soundtrack to Creepshow, and hopefully it will wind up on radio over the next six months. Although it won't go into a lot of detail about Jack Kamen, it still covers the EC horror comic connection.
My website is mostly only doco production info, you can still sign up for a newsletter, which will let you know where and when it will be broadcast.
www.fotwaudio.com 