I talked to a gal who was doing maintenance on a huge living wall at Epcot. She said they have specific plants that they use for the wall, and they rotate them out to a greenhouse to keep the plants healthy, trim them up, get them growing again, and bring them back in to place into the wall again. I'd think some variation on this would be how you'd have to do it with the low light situation you have.
That's probably what the $1000/month maintenance people do, but they just bring you new plants every month. Not very "green" for something that's supposed to improve your environment.
Various types of Coleus are pretty good with low light, and have great colors. They start so readily from cuttings, you could keep a stream of new small plants coming pretty economically.
