Never had a pionus but i use the aviator harness for all my birds its the best one and every bird can learn to like it so i suggest the aviator harness to anyone wanting to take their bird places!
i haven't tried this myself but i really love the products sold at kings cages. plus the "safety leash" sold mimics what is used by falconers. i would probably trust this over trying to get my bird dressed. i think somewhere on youtube or the website is marc marone showing you how to use it.