After building + deploying the hello widget example it didn't show up in Perspective Manager. Engineering identified the issue: the build process does not create a manifest statement.
Workaround for now (assuming that the name of the extension project is "greetings" / "greet":)
Open the file "greet-all.js" in ".../src/bundles" and add the corresponding line manually:
define([
// add public files for this module here
// Application widgets
'greet/widgets/hello/hello.view',
'json!greet/widgets/hello/hello.manifest.json'
], {});