I currently have my mamebox hooked up to a 62" widescreen TV in my secondary TV room. The computer is just another input, so switching to the game is just as easy as picking up the controller and changing to another video input. Initially, I just hooked it up this way as a stopgap measure before I built or otherwise acquired a cab, but I actually like it so much, I might just leave it this way. Honestly, people that visit the house truly love it -- playing arcade classics at sizes you wouldn't even dream of back in the day.
One thing it is not, though, is authentic (although I do use arcade controls). For me, though, it is about the games. With that in mind, I suspect that most people who would be interested in something like this couldn't care less if it is at the authentic resolution or not, and would not want to pay the premium -- or give up the space -- for a mechanism that would spin a big screen on command. Frankly, on a big screen, the vertical/horizontal thing isn't that big of a deal because there's so much real estate -- most people that are not in the MAME community couldn't tell you if a game is supposed to be on a horizontal or vertical monitor, and it might even take a bit of the fun out of it for my guests if I had to jump in every time they switched games to spin the monitor. They are so wowed by seeing Ms. Pac Man at such a size, they don't even notice that it isn't arcade perfect in terms of proportions.
If you're going to market something like this, I'd suggest doing it to non-purists with no skills rather than the people on this board. One problem you're going to encounter, though, is that with most new TVs having a PC port, it just isn't that hard to hook up a mame system to a big screen on your own. As such, if the primary benefit of the thing is that it spins, I'd respectfully recommend that you go back to the ol' drawing board.