To provide a custom component docked into statusbar of NetBeans platform/IDE ( actually I'm using it only as a platform :>) ) just create class implementing org.openide.awt.StatusLineElementProvider. It is a simple one-method interface:
public abstract interface StatusLineElementProvider {
public abstract Component getStatusLineElement();
}
And return your custom component there.
Just make sure you are returning always the same component, because this method is called at last twice on NetBeans start ( at last it was for me ).
When you are done, make NetBeans know about it by writting it's fully qualified name into /META-INF/services/org.openide.awt.StatusLineElementProvider file.
( for source code samples look at ProblemsVisualizerProvider in org.netbeans.modules.autoupdate package in NetBeans Platform source tree. )