A few tidbits from alxpnv, who did a lot (most?) of the work on making Sanitarium compatible with ScummVM.

There is another easter egg in Chapter 7 i.e. "The Morgue and Cemetery" found while poking at the disassembly: in the Aztec temple, click 20 (!) times on the statue of an Aztec Warrior. The angelic statue on the roof will start "breathing."

Out of bounds hack: The whole scene can be explored by entering the scrolling mode: in the ScummVM Debug Console (invoked by hitting Ctrl-Alt-d), type use_scrolling 1 and hit Enter. Now you can scroll through the map by moving a mouse and position the player character by left-clicking.

Another useful addition of the engine is the resource viewer: in the Debug Console, type view # [optional resource id], where # is the chapter number + 4. Space/Backspace are used for navigation. I've found some unused graphics in this mode, e.g. view 7 12.