Rebind Win+F hotkey in Windows 7

Since you mention "hook applications", guess you know about AutoHotKey and the like. You can disable all built-in Windows hotkeys except +L and +U by making the following change to the registry (should work in XP+):

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
NoWinKeys REG_DWORD 0x00000001 (1)

To make +F launch a custom app would indeed require a keyboard hook; I don't believe you can do without one. The Scancode Map REG_BINARY value under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout (which you can edit easily using programs such as SharpKeys) only lets you remap single keys.