¿Acceso al teclado para cambiar el comportamiento de las teclas de función?

Puede utilizar un private.xml así con KeyRemap4MacBook:

<?xml version="1.0"?>
<root>
  <item>
    <name>test</name>
    <identifier>test</identifier>
    <autogen>__DoublePressModifier__ KeyCode::FN, KeyCode::FN, KeyCode::VK_CONFIG_TOGGLE_flipfkeys</autogen>
  </item>
  <item>
    <name>flipfkeys</name>
    <identifier vk_config="true">flipfkeys</identifier>
    <autogen>__KeyToKey__ KeyCode::BRIGHTNESS_DOWN, KeyCode::F1</autogen>
    <autogen>__KeyToKey__ KeyCode::BRIGHTNESS_UP, KeyCode::F2</autogen>
    <autogen>__KeyToKey__ KeyCode::EXPOSE_ALL, KeyCode::F3</autogen>
    <autogen>__KeyToKey__ KeyCode::LAUNCHPAD, KeyCode::F4</autogen>
    <autogen>__ConsumerToKey__ ConsumerKeyCode::KEYBOARDLIGHT_LOW, KeyCode::F5</autogen>
    <autogen>__ConsumerToKey__ ConsumerKeyCode::KEYBOARDLIGHT_HIGH, KeyCode::F6</autogen>
    <autogen>__ConsumerToKey__ ConsumerKeyCode::MUSIC_PREV, KeyCode::F7</autogen>
    <autogen>__ConsumerToKey__ ConsumerKeyCode::MUSIC_PLAY, KeyCode::F8</autogen>
    <autogen>__ConsumerToKey__ ConsumerKeyCode::MUSIC_NEXT, KeyCode::F9</autogen>
    <autogen>__ConsumerToKey__ ConsumerKeyCode::VOLUME_MUTE, KeyCode::F10</autogen>
    <autogen>__ConsumerToKey__ ConsumerKeyCode::VOLUME_DOWN, KeyCode::F11</autogen>
    <autogen>__ConsumerToKey__ ConsumerKeyCode::VOLUME_UP, KeyCode::F12</autogen>
    <autogen>__KeyToKey__ KeyCode::F1, KeyCode::BRIGHTNESS_DOWN</autogen>
    <autogen>__KeyToKey__ KeyCode::F2, KeyCode::BRIGHTNESS_UP</autogen>
    <autogen>__KeyToKey__ KeyCode::F3, KeyCode::EXPOSE_ALL</autogen>
    <autogen>__KeyToKey__ KeyCode::F4, KeyCode::LAUNCHPAD</autogen>
    <autogen>__KeyToConsumer__ KeyCode::F5, ConsumerKeyCode::KEYBOARDLIGHT_LOW</autogen>
    <autogen>__KeyToConsumer__ KeyCode::F6, ConsumerKeyCode::KEYBOARDLIGHT_HIGH</autogen>
    <autogen>__KeyToConsumer__ KeyCode::F7, ConsumerKeyCode::MUSIC_PREV</autogen>
    <autogen>__KeyToConsumer__ KeyCode::F8, ConsumerKeyCode::MUSIC_PLAY</autogen>
    <autogen>__KeyToConsumer__ KeyCode::F9, ConsumerKeyCode::MUSIC_NEXT</autogen>
    <autogen>__KeyToConsumer__ KeyCode::F10, ConsumerKeyCode::VOLUME_MUTE</autogen>
    <autogen>__KeyToConsumer__ KeyCode::F11, ConsumerKeyCode::VOLUME_DOWN</autogen>
    <autogen>__KeyToConsumer__ KeyCode::F12, ConsumerKeyCode::VOLUME_UP</autogen>
  </item>
</root>

Hace que al pulsar fn dos veces se altere el uso de las teclas F como teclas de función normales.

Puede utilizar Palua . No sólo te permite personalizar el atajo de teclado, sino que también te permite elegir qué aplicaciones utilizan qué comportamiento. Los mejores 0,99 dólares que he gastado.