The plants you mentioned are going to be tough to keep with koi. If you have any shallow areas of your pond, you can try planting marginal/bog plants, ie arrowheads, pickerel rush, thalia to name a few. Can also boost pots up on crates, cinderblocks, whatever to get the rim above the water surface, which is key to keep the koi out. Also, could try small floating islands, have plant pockets in them.
You can try the floating ring with net hanging down, but eventually the koi figure it out. Short of creating another area where the koi have no access, there isn't a whole lot you can do for the plants you mentioned.
I have koi in my current pond but none are large, all 12" and under. I have let water celery grow through out the perimeter of my pond, which gives the koi something to eat on. As well, as a seperate area of the pond that I let get over run with plants for years, recently I moved some rocks around to open it up to the koi.