Well at least some of the Iwasaki western straights made with Swedish steel have been heat treated with the Honyaki method.
I personally have not found a hamon on the examples I have in my collection, but to be honest, most of them are still waiting for their restoration, so no hamon could be visible at the moment. But here is a picture So Yamachita sent me some years ago, where the hamon is clearly visible.
Attachment 269663
Anyhow - heat treatment was something Iwasaki experimented a lot. He sent razors to barbers and asked them about the performance. If the barbers gave a feed back that the razors was to soft iwasaki hardenend them again to higher HV grades (as far as possible). Nearly all of the characters and a lot of the numbers you found on Iwasaki razor have something to do with different heat treatment.
After a couple of years that he practiced this field experiment, he recognized that the reason why a lot of barbers sent his razors back to him and claimed, was not mainly a wrong hardening, but the incompetence of the barbers in honing. That was when he wrote his essay on Honing razors and Nihon Kamisories. He also began to sell special quality whetstones at this time. I have read somewhere here in the forum that Iwasaki himself never sold whetstones. That is not true.
I have a friend who is blacksmith in Sanjo city and knows Shigeyoshi Iwasaki and Mizuichi san very well. He also bought a lot of whetstones from the last wholesaler of Iwasaki who ended his business early this year.
Sanjyo Seisakujo, the company of Iwasaki, sold Japanese whetstones, especially Maruka stamped hones for quite a while. But they stopped selling hones some years ago.