Thats one you don't here everyday !! .... What about Dichondra ?

Creeping sedums and succulents would be perfect for this. The link below shows how to construct, although their example uses a tower instead of a globe. Good luck w/your project!

Here is a link that might be useful: Succulent delight for your garden

I'm guessing that when you referred to moss, you meant "dead" moss used as a medium. Have you considered live moss? It's very short, tolerates sun and shade, and if you put a swivel where it hangs, it will be able to get a more even exposure.

I would assume that you would hang it under a porch or tree in a region where it will be misted by a hummingbird feeder, perhaps. Indoors, you could hand-mist it. There are lots of interesting mosses that might work well, but one that has some texture would work best, perhaps. Mosses are often "seeded" by mixing the moss you want to propagate with yogurt, and painting the place where the moss should grow. There's a place I found on the internet recenty which sells mosses.

I think it might help if you put an attachment to your sphere where you could invert it periodicly, to keep it growing more symetrically. That, and the rotating swivel would help make sure that it has a fairly uniform exposure to weather and light. Actually, if you put hanging points every 90 degrees around a great circle then you could rotate it so that the equator could be moved to the pole, also, and that orientation inverted also. This would assure a very even exposure to light and weather. I would think that any extremely short plant that tolerates dry conditions and being potted and part sun, part shade might do quite well. Is there a kind of very, very low phlox? That might be really good.

Very interesting idea!

Perhaps some of the alpine flowers would be very good in this application. They tolerate dry and cold well. The main problem with the ones I'm thinking of is that they grow so slowly, it would be like a bonsai. Your grand-children might be able to begin to see it at approaching some sort of maturity.

I would think that for a long-term substrate for such things, some sort of large-celled glass foam, like you see for sanding blocks, might be a good substrate with strength and durability. It would break if you dropped it, though.

Perhaps the ideal container would be to cast a clay sphere with clay that's infused with short threads, which would burn off in firing to make a way for roots to get inside the pot. I suppose that with some plants, the roots would eventually break the pot. Perhaps instead of the thread, you could just drill a lot of holds in the pot when it's bisque.

Epiphytic bromeliads would be cool (Spanish moss is an example, but only one of thousands and a less showy one at that. But I'd stick to the miniatures). Of coure the old stand bys of Ficus pumila and Hedera helix, but those are fairly common (for a reason; they're EASY) Sedums are a good bet, as are hens and chicks, and those would be cheap and easy.

The ficus and the ivy could become house plants in winter as well (would probably need to to survive, I'm guessing) so that's a consideration. Neat idea, Mike...