I dont know for sure that this is what triggers the females to bellie up or not, I have noted that they alway get full of eggs durining the winter. I just figured that since this happen durning the cold weather that if you reduced the temp for a period of time that the females should start filling up with eggs again. Mother nature is a stange thing and if you mimick her I would think it might work. If I could figure out a way to get the water cold enough and say leave them for about six weeks in cold water I believe that she would spawn again with a slow increase in temp It just makes since to me. To my understanding and I dont know where I heard this but koi need at least 6 weeks of cold in order to do their thing. I mean think about it. Mother nature know what she is doing, and I have alway had good luck with fish if I followed her. For instance when my water starts turning green in my pond the koi will always spawn right before it get so dark you cant see to the bottom. So if I want to do a spawn I would time it with her, when the water starts turning green its time. You may thing this is strange but it seam to work. I have noted two years straight that they spawn when water get nice and green.
If it has been proven to work I didn't do it
Im just thinking and I may or may not have it right. One thing I can say for sure that If I had a way to keep the water cold for a least 6 weeks I would give it a try.