Blahhh, it also looks like toolbar customisations are reset when the view identity changes. It works on iPad so I’m not sure if it's intentional (plus seems like it would defeat the purpose of specifying a toolbar identifier?)
No customisation on the Mac for now then 😢
https://gist.github.com/phillipcaudell/503ea2d3e2e7101b41a44f1c747387ad