Well, I happen to have just learned that springboard bookmarks (the ones you create with Safari) are in this folder: private/var/mobile/Library/WebClips. Each bookmark has it's own folder. And, within each folder, is the .png for the icon. Maybe if you put the new icon there it'll change it? Of course, you'd have to name it exactly the same as the original .png, else the iPad won't "find" it, but I don't see why it wouldn't work.
On another note, there's an app in the Apple Store called "Touch Icon Creator" ($.99 USD) that will allow you to use your own images to create nice icons for your bookmarks. I usually find the logo for the web site and use that as the image. Ends up that sometimes I can't tell which is a bookmark and which is a genuine App Store icon. Of course, YMMV.
Hope this helps - or comes close to answering your question.
Marilyn