This idea is based on the Builder, from RFtools, which can be used to copy a building from one part of a world to another, requiring little more than the blocks needed to reconstruct it, and some power.
This idea is to have a block which does something similar, but instead of looking elsewhere in the world, it reads the .mca files of another world entirely.
Obviously, the config file for whatever mod supplies this block would have to specify the base path to the other world. Also, and this is very important, there would need to be some way to be certain that the other world is offline, without some other Minecraft server reading from or writing to them.
Just like the Builder, the new block would need to be supplied with energy and blocks to build with.