I've been having good luck using sodium bentonite/peat as a moss growing medium, not only does the peat grow, but other mosses that I shred and add in come up as well. You could try to spread a little clay/moss mix on some parts of the bg and see what pops up. I'm working on a similar build right now and will be filling the cracks between the cork pieces with clay mixed with shredded moss instead of stuffing with sphag.