Same as going forward, you'd grab a free block and simply fill it backwards from the end instead of forwards from the beginning. But the file system would have

Same as going forward, you'd grab a free block and simply fill it backwards from the end instead of forwards from the beginning. But the file system would have to support file data starting mid-block and new blocks getting added to the head of the file. The problem is that there'd be more bookkeeping data to store, more code to implement it, and more edge cases to handle for concurrent writes.