It is my understanding that it is not a good idea to add ground cover to bonsai trees. The roots of these plants will compete with the tree for limited nutrients in a small available area to the detriment of the tree. A patch or two carefully kept in check would be ok.
Moss is also not recommended for two reasons: 1) If you cover the whole surface with it, you will cut off much of the oxygen the tree's roots need. 2) If you let it climb onto the trunk of the tree you could kill the tree because, while on a tree in the forest moss is not a problem due to its size, tiny tentacles (roots?) penetrate under the bark. On a little bonsai this can lead to a peeling of the bark and the eventual death of the tree.
Krys

Me again,
I just read my own message and there's an error: When I wrote "a patch or two carefully kept in check would be ok", that phrase was supposed to apply to moss. I inserted it in the wrong place.
I have no idea how agressive the roots of the plants you want may be but you should take great care.
Krys