How can I use the F14 and F15 key for a shortcut?

Hi folks,

I've been trying to assign custom shortcuts to the F14 and F15 keys of my keyboard (wired, full sized type) on my Mac Pro 2008 running Mac OS X Lion 10.7.2 however every time I try to assign those keys to an app's "Record a shortcut" function I get the same error dialog from the OS:

The key combination F15 can't be used!

The key combination "F15" can't be used because it's already used by a system-wide keyboard shortcut. (If you really want to use this key combination, most shortcuts can be changed in the Keyboard & Mouse panel in System Preferences.)

(note: the same happens with F14, F13 and lower as well as F16+ work fine however)

I have looked into my system settings as the dialog suggests however there isn't a single shortcut that uses F14 or F15 anywhere in any of the shortcut groups.

If any of you have an idea of how I could "clear" or even just find out what these keys are currently being used for/by that would be super… they don't seem to do or trigger anything when I press them.

Here is another screenshot of the issue:

Note: This is not app specific, I've tried this in various apps and every time I get the same Mac OS popup.

Thanks for reading,

I look forward to your ideas and insights!