Cobber's hammer --- I've seen people use a cobber's thumb and was wondering if a broad wooden mallet is ever used for compaction. A big mallet with several thumbs inserted into one of its faces might prove to be quick and easy on the hands. For those who like to stand on the wall, a tamper could be similarly altered.
In traditional English cobbing , workers stood on top of the wall and worked in extra
straw with their feet. I would think that shoes with bigger than usual football cleats would help to tie layers together.
Obviously it would be more difficult if the
cob is too wet and gummy. But on a good stiff mix that has sat for a while and can bear some weight , treading is bound to be easier than pushing a thumb by hand.
Best for fairly thick walls (safety) and for systems utilising forms since the formwork would work as a railing so you don't fall off of the wall.
There are 4 pronged
lawn aerators that might do the job. Might need to be weighted.