Hmmm... You probably couldn't go with gooey, but you could do squishy! What comes to mind to me is those kind of sticky squishy balls with the little spikes on them. Do you know what I mean? I bet a bunch of those stuck to the "wall" of the stomach would get you a decent effect.
Take a look at Stilt Beast's youtube video for building intestines. It might work for what you have in mind.
All I'll say is the neighborhood kids will love you for it. My candy bowl last year was a coffin with a skeleton, and the rib cage stuffed full of candy. The kids thought it was the greatest thing in the world, digging into a skeleton to grab a piece of candy. More amazingly, every last one of them only took a single piece of candy!