E.g. if a customobject/structure containing lava spawns underwater in the ocean.

Thinking about this, checking for adjacent blocks could cause issues due to checks across chunk boundaries. Could just make a setting ReplaceLavaToMagmaInWater, which would use magma instead of lava for any lava block that replaces a water block in the world.

Perhaps y'all could genericize that in BO3s to a common SWITCH()-style function, something like this?

ConditionalBlock(x,y,z, Block[:Data], SourceBlock[:Data], AnotherBlock[:Data], AnotherSourceBlock[:Data],...DefaultBlock[:Data])

This way, it would be useful for more than just lava/water relationships.