I managed to hack Castlevania: Lament of Innocence so that the Memorial Ticket teleports the player character to any room of my choosing. There doesn't seem to be any wholly unused rooms, at least in the North American version of the game. There are three areas normally only visited in cutscenes. Using the Memorial Ticket cheat I can visit these areas. Cheats are also used in the video below so that Memorial Tickets and MP are not expended.
Forest Path - Zone 556 (Or World 4, Room 2)
Normally only seen when starting a new game. If you run too far beyond the trees, you'll fall off the edge and into the dark abyss. The two gateways lead to the two ends of the Castle Entrance area, aka Zone 554 (World 4, Room 0).
Rinaldo's Backyard - Zone 558 (World 4, Room 4)
The gravestone is already present even if you teleport here at the very start of the game. I have not found any gateways. There is a pathway that circles Rinaldo's shop/house and backyard. Once you enter that pathway, you can't return to the backyard. If you jump too far out while on this pathway, it is possible to fall off the edge.
Destroyed Castle Entrance - Zone 559 (World 4, Room 5)
Only briefly visited after defeating the game's final boss. Roaming it we can see that the skybox is incomplete. In the endgame cutscene, the camera is positioned so that you'll never see that the skybox is broken. One gate leads to the unused door inside Rinaldo's shop and the second gateway leads you right into the castle, into Zone 484 (World 7, Room 0) as though the castle had not just crumbled at all.
As always, I use Cheat Engine for tricks like these. These are the addresses found in the North American version, played on the PCSX2 emulator.
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20B35C2C: Current World set for teleportation to, using Memorial Ticket
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20B35C2E: Current Room set for teleportation to, using Memorial Ticket
You can't use the Zone value address (20B5E580) for teleportation. Trying to do so will crash the emulator. It is also a fact that every save room has the exact same zone value: 61. The save rooms are thus distinguished by their World-Room values.
20B35C2C values and the respective Worlds:
0 - House of Sacred Remains
1 - Anti-Soul Mysteries Lab
2 - Dark Palace of Waterfalls
3 - Garden Forgotten By Time
4 - Castle Entrance (Forest)
5 - Pagoda of the Misty Moon
6 - Prison of Eternal Torture (Castle Entrance)
7 - Castle Entrance (Prelude to the Dark Abyss)
8 - Ghostly Theatre
9 - Unused save room (for unused placeholder world?)
10 - Unused save room (for unused placeholder world?)
Trying to teleport while the World value is set above 10, or trying to teleport to a Room that doesn't exist, leads to the player character not moving anywhere at all.
In some cases, the player character will teleport inside a wall but still be able to move. They'll be able to position them inside the current Room and then resume normal play. In many cases however, you'll be stuck in the wall and have to reset the game (or load a save state).
One day I'll acquire- and research the Japanese version.