the larval stage of any Lepidoptera are virtual eating machines. I raise 8 species of Saturniid moths (not as feeders) and believe me, it is a lot of work to keep them in fresh food whether inside or out.
You would need to find some safe fruit worm or other caterpillar that is not too picky about its food but they rarely get very large and they often wrap themselves (or stitch together) leaves as they feed. So a neat idea but I really don't think it would be worth the effort unless you have a greenhouse.
I did recently read an article about feeding hornworms non-toxic plants, but even if you were successful in this you would still need a LOT of plant and keep in mind - caterpillars are pooping machines, too. If you don;t clean up their frass and it gets wet fungus, bacteria and virus can easily spread (among the cats)
Here's a link to article on hornworms. hmmm, thought I knew where it was - will have to dig it up for you.
lele
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Chameleon Help & Resource Info
1.0 Nosy Be Panther Chameleon - Cyrus
0.1 Veiled Chameleon - Luna. She's now hanging from her big jungle gym in the sky
1.0 Beardie - Darwin
1.1.1 Side-blotched lizards - Ana and Stan
0.2 felines - Kyndra and Lita
0.1 African Clawed Frog - Skippy
0.1 Chilean Rose Hair Tarantula - Rosa Leigh, Died 4/21/06 
0.1 Goliath Bird-Eater Tarantula - Natasha, donated to science 4/4/06
?.? Pinktoe Tarantula - no name yet