Based on an idea of EricL's for Dna-less plants, I created a veggie species that rapidly deplete's their nrg supply and falls to the bots below as a rain of corpses. Worked amazingly well for a long term sim, since the corpses can't accidentally mutate like the plants can. The pure carnivore seemed to help also, they've stayed largely non-canni for an astoundingly long time, reaching a stable population of 1000 from an intial stable population of 60.
I'm planning to work some more on the concept and see about using it with a gene swapping bot; since the corpses can't accidentally absorb a virus, like plants, it would be much easier to use. It might also be usable for an oddball top down ecosystem sim, with the carnivores on the bottom and nrg tie feeders above them.