I left some review comments.

Since the rules for read-only and write-only access are so different, would it make more sense to have a separate method? Maybe your writefile() name, or open_write() or something? I am not too fussed, but unless there is a chance of being able to open a read-write random access file, I find these split-personality open() methods a bit “un-Pythonic”. On the other hand, I guess it is superficially consistent with other open() functions.

Also, perhaps if you guaranteed the write-only option returned a file-like object, you could use shutil.copyfileobj() rather than a custom read-write loop.