I have a transaction closing in two weeks and another in mid January that were unlisted new construction condos. Both of these homes are in new construction three flats with prices between $400k-$550k. Depending on what my clients are looking for I tend to transact between 5 - 10 unlisted properties annually.
So, the short response to your question is that there are unlisted properties out there, including seven figure homes. But accessing these properties relates more to a realtor's knowledge of an area than a compendium of online information.
Depending on what you seek I think you will be best served by identifying a real estate professional who is familiar with the housing commodity that you want and enlisting his or her assistance to find it. A word of caution, though. If you seek to be in a magnificently massive building well in advance of its completion be cognizant that any downtrending in the market after your purchase may result in your spending much more than later purchasers in the same building.
Best of luck with your endeavor.