You can use FunctionFlip which can 'flip' certain function keys instead of changing all keys.
FunctionFlip individually controls your MacBook or MacBook Pro's function keys, turning special keys back to regular F-keys, or vice-versa.
FunctionFlip's purpose is simply to disable the special features — rewind, play, mute, etc. — on the function keys. For example, if you "flip" F7, F8, and F9, those keys — only those keys — will revert back to normal F keys. Press the fn key with the special key to get the special function back. That is, the "special" and "normal" functions are flipped.
You can also use KeyRemap4MacBook with a private.xml such as this:
<name>F8 as Function</name>
<autogen>__KeyToKey__ KeyCode::VK_CONSUMERKEY_MUSIC_PLAY, KeyCode::F8</autogen>
<autogen>__KeyToKey__ KeyCode::F4, KeyCode::VK_CONSUMERKEY_MUSIC_PLAY</autogen>