Since time is of the essence, instead of a full carpet ('tho, I know that is the look you're smitten with currently) you could toss in a few small moss balls to give some green interest under the tree, and around the rocks. They'd be more in line w/ the needs of the moss in the "tree", and you could get some traction with a lower-tech setup until you're ready to leap to CO2.
Alternately, I've seen in person (but, admittedly not done this myself) very successful patches of "lawn"-like areas created by opening up a large moss ball (it is a rather civilized algae, after all) and affixing it to a large nearly-flat rock. It took a bit of time to adjust and look more natural (less puckered), but it sure was an easy way to get a low-growing lawn-like covering! She had that for a LONG time - it was very stable and easy-care, as long as it was tended by either some Amano shrimp, or, later, she had a solitary siamese-algae-eater-relative who tended his personal garden very happily (didn't like conspecifics much, so everyone was happy). The need to "tend" it a bit was to prevent other opportunistic algae from moving in.
It was actually quite lovely in a grouping of several large flat, clado-covered low islands, and continued to look really good for several years - she finally changed it up from boredom rather than any problem with the carpet-rocks.
Just a thought.