It could be dangerous, or at least uncomfortable. Newer style tree saddles incorporate a hard bosun type seat that is worlds away in comfort from rock climbing harnesses or older tree saddles. No longer do they painfully cut off circulation to the legs or, um, other parts of the body. There are many good mail order outfits which sell tree climbing equipment. You can learn from an experienced climber, or through professional training.
Experienced climbers can be trained arborists, or members of a climbing club.
Anyone who goes into a tree with a rope and a saddle should be familiar with ANSI Z133 standards, which cover safety procedures as well as equipment specifications. Cooperative Extension s Community Forestry Program promotes the establishment and maintenance of healthy trees in the human environment.