The fallback world is described in the readme as:

Optionally, it can also use an existing single player world to load in chunks which the server has never sent before. To make use of this feature, simply rename the world folder (not the name of the world! the name of its folder!) to "bobby-fallback".

Does this really mean there can only be one fallback world for all servers?
How about naming it like bobby-fallback-exampleserver.com:123, so that each server can have its own matching world?

What might be ideal is a mention in the README that you can drop in the region files from a save into the .bobby directory for that server.

Well this would be useful to be able to generate fallback chunks if you know the world seed.

It would be nice if it was just within the bobby server files, like ".minecraft.bobby\server.ip.address\bobby-fallback", is that not very easily possible? Would be great if that could be done, so you can give your friends a seeded tiny world file and their computer will fill in the rest.