In Terminal, do:
sudo vi /System/Library/Extensions/AppleHIDKeyboard.kext
Go to line 349, which is where the function key definitions reside. I haven't fully decoded the system yet, but I think that it works like this: Key,Function. For example:
0x0007003a,0xff010021
0x0007003a is the F1 key without the "fn" key pressed, and 0xff010021 is the code to decrease the screen's brightness. Based on that, here's my ugly hack. I changed 0x0007003d to 0x0007004d -- this apparently made the system think I pressed fn-F4 or something. Either way, it works, and it shows the desktop as I wanted it to.
Save the file and reload the driver with:
kextunload /System/Library/Extensions/AppleHIDKeyboard.kext && kextload /System/Library/Extensions/AppleHIDKeyboard.kext
[robg adds: I haven't tested this one; if you're going to do so, proceed with caution, given you're changing a system driver file.]