I agree that is most circumstances you will not get back 100% of what you spend on a project but if you plan to enjoy it for many years before selling, the cost of building continues to climb and will in time make up the difference, plus you get those years of enjoyment.
This will be fairly difficult to accomplish becuase of how the MLS stores room size information. Rather than calculating that a room is 100 square feet, an agent inputs that the room is 10 x 10. The type of data analysis could be done but would take be very in depth knowledge of the MLS export capability and further a decent knowledge of MS Excel. I doubt that this is ever something that Trulia would be able to accomplish but I would be impressed to be proved wrong.
By ROI one normally implies that they are hoping to make money off of something. Except in very rare circumstance can someone "pop the top" (add a second floor) to a home and make any sort of money off of the deal. It will certainly add value but unless you plan to do all of the work yourself you will rarely return what you spend, and if you do the work yourself you are the time and hassle.
As a residential designer as well, I would caution you against playing with too many cantilevers as you don't want to home to appear top heavy simply to add square footage. I would be happy to look at your ideas and give you my thoughts about the design if you would like.
I can do the data analysis if you would like. Let me know.