That's a good question. I know beans carry the toxin, lectin, but I'm not sure if this is present in the leaf or just the beans. What is making you lean towards the green bean plant?
I have the seeds in my hand and they grow fast. I'm sure my mom has some other plants but this was all I was familiar with (we do lots of gardening in the summer).
Right, I understand wanting to use something that is a fast grower. I use Hyacinth Bean Vine in my amphibian vivariums for this exact reason...plus they have awesome purple flowers!