You don't need to hack anything ;)
I am not entirely sure why you need the data on mysql. If I knew, maybe there would be a more suitable answer. In any case, as a generic answer you can use redis keyspace notifications
You could subscribe to the commands HSET, HMSET, HDEL and DEL on your keys, so you would get a notification everytime a key is deleted or a hash value is set or removed.
Note if you miss any notification you would have an inconsistency. So once in a while you could just use the SCAN command to go through all your keys and check on mysql if they need to be updated.
Another strategy could be maintaining two separate structures. One would be the hash with the values, and the other would be a ZSET of all the values sorted by timestamp of update. The best way to keep both structures up to date would be to write two or three lua scripts (insert/update and delete) that would operate on the hash and the zset atomically.
Then you can just periodically query the ZSET for the elements with a timestamp higher than your last sync operation, get all the keys that were updated (it would include deleted keys, unless you want to keep a second ZSET exclusively for those) and then just retrieve all the elements by key and sync to mysql.
Hope it will work for you!