It's actually fully functional in Banjo-Kazooie, it wasn't until Donkey Kong 64 that Rare was told to remove it from the game. The game is constantly writing to a certain region of memory with information about which hidden locations are unlocked, then on the flipside when the game boots it checks RAM to see if the special data is there, then unlocks the locations. It doesn't actually work by hot swapping, it relies on the fact that the RAM is preserved for a certain amount of time after shutting the console off.
I realized recently that you could actually test this -- with a sufficiently old N64 console, and two copies of Banjo Kazooie, you could use the cheat codes to unlock the hidden locations on one game, then swap over to the second copy. Theoretically, that should unlock them without having to use the cheat codes.
If I was a bit more motivated to spend the money, I'd try it myself and make a Youtube video about it!
e; actually I think it's not possible for a Banjo-Kazooie game to transfer to another Banjo-Kazooie game, the memory that it writes to is different from the one it reads from. https://tcrf.net/Banjo-Kazooie/Stop_N_Swop
