Nothing.
I'm almost certain that even wih a hack, you would not be able to enter Challenger's Cave. The Pokemon games work by using map tiles to warp you to another map. If you step on a tile, if it is coded to warp you to a different map, it will. However, if the tile is not coded to warp you anywhere, it won't warp you anywhere. Pretty simple.
So if Game Freak removed the warp in Route 9 that took you to Challenger's Cave, then all a hack would allow you to do is stand in the wall where the entrance once was. If the warp is still there (unlikely), then whether or not it take you to Challenger's Cave depends on whether or not the map for Challenger's Cave even exists in the game. If Challenger's Cave does not exist, then the warp would either take you to a corrupted map or simply not work.
It's similar to how a wall hack won't allow you to access Route 10 from Opelucid City, because there is no warp that takes you to Route 10 any more. Again, I'm not 100% sure if Route 10 is still in the code though, if it does then you may be able to access it through a different hack.
