In Red or Blue, there is something known as Glitch City, which is just a misread warp from a glitch you attempt in the Safari Zone. In Generation IV, if you walk out of bounds, instead of the game crashing, it would instead take you to the Mystery Zone, which are supposed to be error handlers surrounding real routes. So pretty much there is not a lot of Hidden Interiors Universe out there in Pokemon. Most of the time the game would just crash if this is attempted.
Resources: http://glitchcity.info/wiki/index.php/Out_of_bounds
