Actually, Cody’s script above can be adapted to be a bit less fragile without too much trouble.
client = __client
mounts = __client.mounts
mainViewId = `${mounts.center.viewPath}@${mounts.center.code}`
mainView = client.page.views.get(mainViewId)
mainView.custom.read("key") // or readArray, readDataset, etc
Still somewhat fragile, but not as bad. PropertyTree on the frontend has various methods to read values from the model as different types. Custom properties are currently ‘synced’ to the frontend, though as Cody mentioned, that default may be changed in the future, since outside of automated testing there’s little point in synchronizing custom properties to the frontend.