I have a trivial module which detects if the current LookAndFeel is Aqua and if so then installs Quaqua 
LooAndFeel.  Quaqua is a proxy LaF which delegates most of its work to Aqua LaF but reimplements 
some controls to fix UI bugs in Apple Aqua LaF.   The notable one is the file chooser.

In essense what the quaqualaf module does is to have this piece of code in its ModuleInstall.restored()

    public void restored () {
        String laf = UIManager.getLookAndFeel().getClass().getName();
        if ("apple.laf.AquaLookAndFeel".equals(laf)) {
            try {
                UIManager.setLookAndFeel(
                    "ch.randelshofer.quaqua.QuaquaLookAndFeel");
            } catch (InstantiationException ex) {
                ex.printStackTrace();
            } catch (UnsupportedLookAndFeelException ex) {
                ex.printStackTrace();
            } catch (ClassNotFoundException ex) {
                ex.printStackTrace();
            } catch (IllegalAccessException ex) {
                ex.printStackTrace();
            }
        }
    }  

This is similar to what Petr Nejedly did a while ago in his contrib/plafswitcher module except quaqualaf 
does it only for quaqua and includes quaqua.jar library.

This module worked perfectly well in 4.0, 4.1 and probably all older versions of NB but does not work 
anymore in 5.0dev builds.  We changed the startup sequence.  The core now sets LaF after all 
ModuleInstall.restored() are called.

Two points

1) this is compatibility breakage

2) it seems there is no way to achieve the same thing in 5.0: a module wants to package a LaF and sets 
it when the IDE is run.  This was possible in all previous versions

I'll attach the NBM and the code of quaqualaf module.  On non-Mac platform this bug can be 
reproduced by a similar module which sets LaF to the non-default one in its ModuleInstall.restored(), ie 
to Metal on windows, motif or gtk on unix