If you want "100%" to mean "actual size", you would have to mess around with your screen dimensions. If you can create a screen preset that shows up in System Preferences -> Display, that gives you the "correct" ppi value, it would probably work.
One caveat though: applications are a bit iffy as to what they mean by "100%". In Photoshop, the percent value has nothing at all to do with size. It's merely a number to tell you how many of the original pixels in the image that actually show up on your screen. At 100%, every pixel in the image is visible, regardless of the resolution. Mind you, they may not all be ON the screen, if you have an image with more pixels in it than your screen can handle.
One would like to think that 50% would mean that you see half of the pixels, but alas, no such luck. The percentage only takes one dimension into account, so 50% actually means that 25% of your pixels are visible.
Photoshop is a strange beast, but it is'nt the only one with strange notions of how to interpret percentages in terms of size.
So, to summarize: your chances of getting this to work properly are slim at best, and realistically non existent. Strange really, since this has been a problem in DTP for twenty years, and noone has bothered to solve it.