I was the one who saw a naturalistic setting at the zoo. they didn't have cactus, rather they had clumps of grasses, some with seed heads. I called the zoo, and the herp keeper told me she digs up grasses from around the zoo. She couldn't tell me what they were, as they don't work on the landscaping.
I got potted tapestry amaranth and lemongrass, both edible by humans & animals. I trimmed the lemongrass and put the pot in my uro enclosure. The amaranth was too tall, so I harvested it and wired the shortened grain stems to a stand to keep it upright. My uros have been climbing & clawing the plants, but I haven't seen evidence of snacking.
I have been researching cacti as well. It's very hard to find small specimens that are known to be safe. Prickly pear grows slowly, although it's hard to grow from seed it can be rooted from a cut mature pad. Uros can eat both the pads & fruit (although plants in pots don't usually bloom). Prickly pear pads can be bought in most local mexican produce market, where they are called nopales. From what I read online, a pad should be planted about a half-inch into cactus potting mix, given little water and kept out of sunlight. After about a month it's roots should be strong enough to keep itself upright, so until then the pad can lean against wooden chopsticks stuck into the soil. In a few months I should have a prickly pear strong enough to withstand a climbing uro. I will let you know how it works out.
Good luck.
--Benedita