Over the years we have visited many koi businesses and koi farms around the USA and in Japan....one thing I noticed a few years ago when we were at Momotaro in Japan...that he was using liner for some of his huge mudponds....I had until that time never seen liner mudponds in Japan....interesting talking with Mr. Maeda at his noodle restaurant about liners...and why...he mentioned "experiment"....sounds now like a very good experiment!!!
Out here I have seen an alternative to spending lets say $1500 for a liner of 30 x 100 ft....there are folks out hre that are having success at using Visqueen...cheap plastic to hold water in a pond...something to consider if you don't have good clay or you are on a tight budget. You will with a bit of luck get a few years or more out of this inexpensive liner...just an inexpensive way to add more water to grow koi and get rid of that pain in the grass...grass...