Has either of the major manufacturer/importers EVER made any scale Pacific other than the USRA engine and its variants (the Southern Ps4 is basically an improved USRA copy) or the Pennsy K-4?
Southwest Hiawatha
You might want to read that original post again. The MTH CNJ G3/P47 Pacific is a very accurate model of a great looking locomotive. MTH has offered it in every prototypical road number and paint scheme for the CNJ as well as using it as a stand-in for other northeastern Pacifics.
Prototype Pacifics could be divided into two categories, pre-USRA designs and post-USRA designs.
Most pre-USRA Pacifics were rather small and light. They were often hand fired. As steel heavyweights became the standard for first class trains these early Pacifics became less suitable for mainline service. At the same time that they were bumped off of premier trains new highways and automobiles were dramatically reducing the demand for branch line passenger trains. Many pre-USRA Pacifics were scrapped in the 1930s and those that made it to the WW II years were likely headed for the scrap man soon after VJ day.
As for post-USRA Pacifics, it is worth noting how many were variations on the USRA design. As you noted the Southern Ps4 is a design that was at least influenced by the USRA heavy Pacific. The same can be said of the B&O P-7, C&O F-19 and Erie K-5A.
The B&O, C&O, CNJ, Erie, Pennsylvania and Southern are all notable as railroads where Pacifics remained in service on their premier trains in the 1940s. By that time most other large railroads had moved on to Husdons, Mountains, Northerns and diesels for their best passenger trains.
Diecast model makers have to find a market for several hundred and preferably a few thousand locomotives to have a profitable project. The market for pre-1940s or pre-USRA era models is rather small. Do you have any Pacific type locomotives to suggest that might have a diecast sized market appeal that have not already been tooled and produced?
One possibility is a New York Central K-5, stepping stone to the Hudson.