Actually is the only actual default registry for global packages.
The jenkins no proxy value is validated against the default npm registry URL, so if you add npm.registry.org in no proxy Jenkins settings the http_proxy and https_proxy are not set during npm install -g action.
If I understand right only npm >= 3 support no_proxy, and only as enviroment variable (details here)
In real it's a little bit complicated becuase some global package could have scope that refer to a different registry:
I should support npmrc in global package and taking care about all registry and foreach one check against jenkins no proxy and make a mix behavior to support also old version of npm.
Anyway remember that you could setup registry and no_proxy as global variable prefixed by npm_config_ as described in my previous post. All enviroment variables with that prefix are considered by npm like written in a .npmrc file.