....

(phew!)  Happy to.

I'll give it some more thought, but don't see any
immediate inspirations 'jumping out at me'.  [ AFAIU -
you know a load more about some aspects of GUIs/GUI
programming than I do, so I doubt I will be the source
of the arcane facts you are seeking, in any case! ]

I still feel, though I might not have clearly stated, that
this thread would get better information on c.l.j.gui.

It has to be added to the content pane before you call your getParent()'s
I gather you have no control over this, just the Panel. Perhaps a timer
might be one way.

Something like this

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
class Testing extends JFrame
{
 public Testing()
 {
   setSize(100,100);
   setLocation(300,200);
   setDefaultCloseOperation(EXIT_ON_CLOSE);
   JTabbedPane tp = new JTabbedPane();
   TabPanel p = new TabPanel();
   tp.addTab("tab 1",p);
   tp.addTab("tab 2",new JPanel());
   //p.setDefault(p.getParent());//if no timer - NPE here
   getContentPane().add(tp);
   //p.setDefault(p.getParent());//if no timer - OK here
 }
 public static void main(String[] args){new Testing().setVisible(true);}
}
class TabPanel extends JPanel
{
 JButton btn = new JButton("OK");
 javax.swing.Timer timer;
 public TabPanel()
 {
   add(btn);
   btn.addActionListener(new ActionListener(){
     public void actionPerformed(ActionEvent ae){
       JOptionPane.showMessageDialog(null,"OK");}});
   ActionListener al = new ActionListener(){
     public void actionPerformed(ActionEvent ae){
       setDefault(TabPanel.this.getParent());}};
   timer = new javax.swing.Timer(100,al);
   timer.start();
 }
 public void setDefault(Container c)
 {
   if(c.isVisible())
   {
     if(c instanceof JRootPane)
     {
       ((JRootPane)c).setDefaultButton(btn);
       timer.stop();
       return;
     }
     else setDefault(c.getParent());
   }
 }
}