So we were more than happy to recently stumble upon WideNES, a new emulation project that extends the visible area of an NES game map past the usual 4:3, 256x240 resolution screen area we're used to. Using WideNES, you can even zoom out and pan through previous screens using the keyboard and mouse wheel while still playing the game.
The generalized methods used by WideNES are described in a fascinating blog post by University of Waterloo student Daniel Prilik, who created the algorithm. Basically, the WideNES system watches the background "map" data constantly scrolling through the emulated NES PPU as you play. It also closely monitors the PPUSCROLL register, which controls the actual screen scrolling by pointing to different parts of VRAM as the player moves.