Unfortunately, the book Sign Languages, Cambridge Language Surveys series, 2010, edited by Diane Brentari, which has the “Variation and change” section, doesn't cover PISL, it is about modern sign languages of the world, and in most books written in the 19th and 20th centuries specifically about PISL nothing is said about its varieties or dialects. But Universal Indian Sign Language, 1936, by William Tomkins, pages 94–95, cites some accounts on the spread of PSIL and its understandability among different Great Plains nations, italics is mine:
Many of the Indians, in a variety of tribes, have stated that in former times the sign language was the one common and universal means of communication between all the tribes of American Indians who spoke different vocal languages. As he expressed it, "All the old people in all the tribes used it."
Little Raven, the former head chief of the Southern Arapahoes, said in regard to the use of gestures: "I have met Comanches, Kiowas, Apaches, Caddos, Gros Ventres, Snakes, Crows, Pawnees, Osages, Arickarees, Nez Perces, Cherokees, Choctaws, Chickasaws, Sacs and Foxes, Pottawattomies, and other tribes whose vocal languages, like those of the tribes named, we did not understand, but we communicated freely in sign language."
"The summer after President Lincoln was killed we had a grand gathering of all the tribes to the east and south of us. Twenty-five different tribes met near old Fort Abercrombie on the Wichita River. The Caddos had a different sign for horse, and also for moving, but the rest were made the same by all the tribes."
Nichelle, chief of the Pend d'Oreilles, said: "All the tribes talk in signs when they meet if they cannot understand each other's vocal language. The Blackfeet, Crows, Flatheads, Kootenays, Peleuses, Cayuses, Pend d'Oreilles, Coeur d'Alenes, Spokanes, Nez Perces, Yakimas and others all make the same signs.
In the record of Major Long's expedition, of which he wrote in 1822, it tells how on his way down the Mississippi a number of strange Indians came into his camp, and Mr. Nolin, who was present, addressed them in such of the languages as he was acquainted with and was not understood. He then conversed by certain signs. These were fully understood by the Indians and were answered in like manner. Directly a conversation ensued in which not a word was spoken. "This," said Nolin, "is a universal language common to the Western Tribes."
As you can see, the variation among different nations was minimal. As for Navajo, they had their own distinct dialect, references are here at the end of “Geography” section.
