Think of it as playing a game with someone sitting right next to you...but they're not. They're just remoting into your desktop from their house, and interacting with the emulator you've got running on your computer.
Like I said, I don't know how well it'll work, or even if it'll work at all. You'll have to account for lag, controller support, and all that good stuff. But if it ends up working well, then you've got your answer for playing old emulated games over the internet.