Best way - if any to "thin out" overly salty kimchi - already in jar
Actually, your idea is pretty much exactly the method used in Korea to mellow out salty kimchi. Apparently it works via osmosis. Link in Korean
Just make sure to spread out the daikon, and let it sit for a few days before checking so that it can get the job done. Also, don't salt it beforehand (although this is admittedly quite obvious).