I don't know about the Plastic Man cartoon (to be honest, I didn't know that there was a Plastic Man cartoon until I read this post), but the Plastic Man comic book did feature foot tickling on at least one occasion, in which Plastic Man tied up a bad guy with his body and tickled his bare foot to get some information. There used to be a web page that featured a lot of comic tickling images (circa 2000 or 2001) that showed this scene, but I cannot find it now.
As for the Spider-Man comic reference, that scene is in Spider-Man #13, volume one, August 1991. And yes, I am fully aware that by providing this degree of detailed information, I am confirming once and for all that I am a complete and total geek. 😉
rwd