I had two weathers that I was breaking to harness but never quite got as far as goat carting before they started getting quite aggressive and I decided to put them in the freezer as I felt it was too much of a risk with my two young children.
They seemed to accept harness much better than harness and I got them harnessed as a pair, dragging a tyre. I used modified nylon foal head collars and cobbled harnessed out of old leather pony tack. I used old leather girths as breast straps as they were padded, strong and had the buckles to connect other tack parts easily.
I walked them in harness many days and never had any rubbing or sores; I think goat hide is pretty tough compared with ponies.
They seemed to really take to the pulling part but didn't progress with voice control as well as I'd hoped. They'd listen to "Pull On!" And get going strong but a calm "steady woah" only resulted in a slight check of speed so mostly I was relying on the reins which isn't ideal. I'd decided to get or make tiny bits for them instead. Then my baby got ill and took up a lot of my time and after watching one head butt my toddler then knock over my mum I knew it wasn't working out so they went in the freezer.