In many cases, buttresses are not the optimum solution for bracing earthbag walls. Post and beam designs are often preferable and have many advantages. Besides being less obtrusive, post and beam designs are suitable for wider range of styles, require less plastering, take up less space, and are faster and easier to build in many cases. Plus, exposed timbers look fantastic! Using poles from a forest obtained with a low cost firewood permit, this option doesn’t have to cost a lot.
You could bury posts within the wall, next to the wall or even set them apart from the wall. One option is to use round posts of say 6”-8” diameter and cut 2” off one side. This provides a flat surface to go against the wall, making plaster work somewhat easier by eliminating recesses behind posts. This also enables you to stack uninterrupted earthbag walls, which is faster and easier than working around posts.
How to connect posts to the walls? Easy. Cut strips of expanded metal lath as ties. Nail one end to the back of the post, bend to 90 degrees and pin to top of bags. You could use the same technique using scrap metal. For pins, strawbale builders make U-shaped pins out of heavy duty galvanized wire.