You can use a widget to deploy a new feature to an existing Notes client using a combination of the widgets catalog, an NSF-based update site in which the feature resides, an XML file attached to the widget that calls the feature's update site, and Domino policy or Eclipse preferences.
This technique is described in Lotus Tech Note 1305829 entitled "How to use a Notes widget to deploy a new plug-in to an existing Notes installation" at http://www.ibm.com/support/docview.wss?rs=899&uid=swg21305829.
Using this procedure, users are automatically provisioned with the feature called by the widget's extension.xml. The feature is installed when the user's local widgets catalog is updated. Updates you make to the feature are provisioned to the user automatically, via the widget, when the user's local widgets catalog is updated.
You may also watch the webcast on Deploying Plug-ins and Widgets for Lotus Notes and Sametime