Hi NocD,
Have you tried just Cmd + F2? I thought I'd have to use the Fn button, but I didn't. That keystroke prompts a script called "dpd" which then puts OS X into sleep mode and uses the display to extend your laptop's display. Naturally, the cable you use has to be a Thunderbolt cable and not a MiniDisplayport cable. I don't know what the difference is, honestly, except for cost.
This worked for me two times but I can't get it to work, now. Now, I use the Cmd + F2 shortcut and the "lib exec" script pops up in the dock toward the right. When it worked, I saw this script pop up (in a little folder icon) and then the display switched over perfectly. Now, I launch the script and the folder just sits there. I have to go into Force Quit Applications to shut it down. It's always listed as not responding, now.
It seems like it should work for you, assuming you have the right cable. Try the keystroke without the Fn button? Change Thunderbolt ports? Try a different cable?