Growing moss

Cakes

Providing shade for moss might be easier if you used a landscaping cloth or shade cloth of some type. Some materials are rather gossamer like and may not crush the more delicate mosses. If you wanted shade but not cloth then you could make a little shade from a strawberry basket-type cage placed over the moss. Shade cages could be made from anything and if a strawberry basket lets in too much light, then simply lay a bit of grass or something on the cage to create more shade.

shade cages are the best thing to use when transplanting veggies and such too; 3 days of shade and the transplant lives every time.

If you have the inclination, there are also solar powered fish tank pumps available; so rigging an intermittant mister out in the garden wouldn't be too difficult (solar pump $15 USD).

Depending on your tree's humidity tolerances, it may also be possible to put the whole thing in a terrarium (especially if you were getting ready for a show?).

Or even this~
wicking moisture to the underside of the moss?
cotton wicks or panty hose placed under the moss might provide a little extra moisture but still not drown some trees. Perhaps the water to be wicked could come from a pot that we place the bonsaii's pot into? the moss could cover it all?

I wonder what kind of value there might be in pruning moss? cut the tips off and they turn color? I can even imagine creating the illusion of a village with crop lands and such under a bonsaii tree.

might moss alter it's color depending on pH?

it prolly wouldn't be a problem to alter the color by feeding it plant dye.