Not near impossible if you know what you're doing. Problem is, most of the people with the means to develop their own have the means to get better screens that actually work without having to develop anything for it.
There're 3.5" and smaller screens on eBay, just search for car reverse monitors. Something like "TFT reverse LCD" and you'll find plenty, and most of them are reasonably priced. If you need it fast you can pay an extra 10 bucks or so and get an American retailer of them, but if it's coming from China expect about a one month wait