Instead of repainting it, close the menu and reopen it. Just tried it and it works great, it's as quick as repainting on the spot.
jMenu1.add(jMenuItem);
//jMenuItem.repaint();
//jMenuItem.revalidate();
MenuElement[] selectionPath = MenuSelectionManager.defaultManager().getSelectedPath();
MenuSelectionManager.defaultManager().clearSelectedPath();
MenuSelectionManager.defaultManager().setSelectedPath(selectionPath);