In looking through Elmer Kieth's book Hell, I Was There, I have only found one reference to the development of heavy loads in a .44 special or .45 Colt. On page 125: "So I then (in 1927 I believe it was) designed the Ideal .429-421, a .44 Special. I found out by then that I could load a .44 Special much heavier and to give more power, more velocity and energy, than anything possible out of the thin-cylinder .45 Colt."
In Elmer Keith's book Sixguns copyright MCMLXI (1961?) on page 44 he writes: "We have long urged that the same fine Magnum [a reference to the .357 Magnum] be brought out in .44 Special caliber and have tried without success to have various loading companies turn out a Magnum .44 Special with the Keith 250 grain bullet backed by 18.5 grains 2400, but so far to no avail. They are afraid of the old type case protruding primer pocket and thin head which they could easily redesign and bring out a solid head. They are perhaps afraid of he old Triple Lock as being made of inferior steel, but we know of a great many of these fine old guns that have successfully digested my heavy loads for a great many years with no casualties."
Keith writes on page 45 of Sixguns about the strength of cylinders: "One thing that adds greatly to the strength of these two little undercover arms is the fact they are five-shot, hence the bolt cut or stop in the cylinder comes between the charge holes, not over the center of them as is true of all six-shot arms."
I was unable to find a clear reference in either book to determine whether Keith was limited to the use of the SAA Colt in developing his heavy .44 Special loads. This is not to say that he did or didn't, only that I could not find a passage in which he stated what he did. Keith wrote many articles which I have never read which may have addressed this.
Regardless, the previous posters have pointed out that the thinner walls of the .45 Colt versus the thicker walls of the .44 Special was the determining factor in why there was not .45 Magnum developed at that time. That time being during the 1920s over two decades before the forming of Sturm, Ruger & Company in 1949.