This might not be that useful, but i remember Jeb saying that Spiders can climb walls like that because they are essentially covered in ladders. Perhaps there is a way to do the same for Creepers?
All the chasing functionality of a spider, with the damage output of a creeper? WHAT HAVE YOU DONE! Awesome, but you know when skeletons ride spiders, do the spiders work as they usually do? Can they still climb walls and stuff? Also, OP sorry for detracting from your post, but think of it as valuable research and bumps EDIT by Moderator: merged posts, please use the edit button instead of double posting.
Hahaha, Bukkit show up with this API! Would it even be possible? I dont think so? Spiderjockey's are also a bug right?
Spiderjockeys are natural. A spider has a 1% chance to have a skeleton on it's back. I think it would be possible, when there would be a way to find out whether a mob is trying to walk but there is a wall.
Theres a Spiderjockey plugin im gonna try and just make a chance for spidercreepers to spawn. Love the rage comic.
Fuzze, there is also Essentials, Not sure which one, but you can type /spawnmob creeper,spider and it will spawn a creeper on a Spider. Edit -- Nevermind, you want them to spawn on their own.