I would steer clear of paint and glue, even if they are non-toxic. They're safe enough for a human to eat some and not die, but I don't think anything like that would be safe for pigs. The flour paste probably won't do them good I agree, and if they're like mine they'll wait til you're out and eat tons of it!
What would be really cool would be to do the theme on the outside of the cage. You could make up a background for the wall behind the cage, so that when you look at the cage it looks like they are in the amazon! Just make sure they can't reach any parts of the decorations and pull them into the cage...pigs are extremely good at doing that!
Let us know how you get on, it sounds like it will look really cool. You could use jungle or leaf patterned fleece in the cage as tents or accessories!