According to this there were studies done in Uganda where Tithonia diversifolia (mexican sunflower) was the sole feed for rabbits successfully, but I don't know enough about nutrient content.
I also heard from someone at an event taking place at H.E.A.R.T. in Lake Wales, FL that they had heard of people feeding exclusively water spinach (considered invasive in water ways, but grows well and is also edible for humans in contained systems) and being quite healthy.
Not really looking to have one plant as a sole feed, just don't want to give too much of some vitamins, nutrients, etc. that might cause any health issues for the rabbits - hairballs, ulcers, eye problems, and so on. I know sometimes improper diet can lead to GI Stasis which is almost always fatal because by the time you realize they have it, they're too malnourished and sick to bring back from it.