I think that this would be a rather straightforward change from the design point of view. On startup, InnoDB would look for both the doublewrite files (I would consider using one file for each physical page size) and the built-in doublewrite buffer. During operation, InnoDB would only write to the doublewrite files, never to the built-in buffer. I would make the doublewrite file names hard-coded in the datadir root. I do not see any reason to make them configurable. If someone wants the files to be written to a different file system, he could manually create the files elsewhere and let symbolic links point to them from the datadir.