I believe it is ok to remove the charcoal completely and pack sphagnum moss around the roots before tying it to the tree. After that the shade, rain and fertilising will take care of the rest.
So it really depends on your growing area, but I can tell you that most gardens are absolutely ideal for growing orchids on trees or non-pot style. So far, my experience with tree-tied orchids have never yielded much improvement as my garden tends to get hot and dry especially during a stretch of days with no rain, so the plants have problems with leaf growth and flowering. Now it is only with plenty of sphagnum packed into the plastic pots between the roots did I manage to get my phals back on track. As a rough idea of how dry my garden is, if you water let's say in the morning, followed by the hot spell throughout the day, and in the evening the top portion of the moss is bone dry and the subsequent layers are just barely moist.
Ferns like maidenhair wilt like crazy in this horrible dryness..Anyway, I'm sure the others have a better view of this growing method.