とりあえず叩き台としてコード書いてみたよ。

tailFile n h = do
　seekable <- hIsSeekable h
　if seekable
　　then do
　　　sz <- hFileSize h
　　　hSeek h AbsoluteSeek $ max 0 (sz - n)
　　　hGetContents h
　　else do
　　　takeLast n <$> hGetContents h

takeLast n xs = diff xs (genericDrop n xs) where
　diff xs [] = xs
　diff (_:xs) (_:ys) = diff xs ys