The whole point of flushing is to remove nutrients from the growing medium. Flushing with coconut water would still add some nutes back into the medium. 15ml/L really isn't much so it wouldn't be too many nutes being left behind, but at that point what point is the coconut water serving? If its to feed microbes I guess it wouldn't be a bad thing, but idk. I'm also in the camp that end of grow flushing is 100% pointless. I've done grows with and without flushing and the quality is exactly the same. The dry and cure are what makes or breaks the quality of your bud, not flushing out nutes and starving your plant. What I've started doing has been slowly tapering my nutes down to about 1/4 strength by the end of flower starting a couple of weeks from harvest. I have to refill my res 2x a week so I usually drop the nutes I add to my water by about 1/4 each refill except the last one which stays at 1/4 strength. The reason I've started doing that vs feeding straight to the end is that I've noticed the plants themselves start eating less and less as they approach being ripe. That's one reason why flushing is not a great thing. The plants are still hungry and want those nutes to grow, they just do it at a slower pace than earlier in flower.