I have not done this, but you should be able to use a "Dummy" plug to trick the nMP into thinking a second display is connected (prob easiest to use TB-DVI adapter to connect the "dummy"-- might not be possible with TB directly).
You would then simply use Screen Sharing to connect to the dummy/phantom display (GigE or TB data connection would be fastest).
Not sure if you'd find a dummy plug to match rMBP native resolution, but you should be able to get something that scales nicely.
Edit: You got me curious, so I looked for DVI dummy plugs. Looks like your best bet might be a cheap HDMI switch. Should get you to 1080p painlessly. Not sure if they would all word, but I work start with a cheap (<$20) one from here: http://www.monoprice.com/Search/Index?keyword=hdmi+switch
Oh. I thought you meant as an ADDITIONAL display. Sounds like you just want to configure a headless mac. That should be possible without any hardware just using Screen Sharing. Google around for instructions.
IIRC, you should be able to SSH into a virgin Mac and enable screen sharing on the new Mac. You might need to monitor your Airport (or other router) to see what IP address it hands your nMP on bootup, or you could use internet sharing from your rMBP (Share Airport to TB-Ethernet port, and plug ethernet into nMP).
I think plugging in a wired mouse and keyboard to the headless nMP might be even easier/faster for you (this apparently will allow Screen Sharing, which should cause the nMP to appear in Finder on your MBP under SHARED).