Not sure about the Nagarjuna connection, but there are a number of other sources which appear to tie the Tibetan culture with the Native Americans.
“When the iron bird flies, and horses run on wheels, the Tibetan people will be scattered like ants across the world, and the Dharma will come to the land of the red faced people”
--(Padma Sambhava, eighth-century Indian guru and founder of the first Tibetan monastery)
"When the iron bird flies, the red-robed people of the East who have lost their land will appear, and the two brothers from across the great ocean will be reunited." --Hopi Prophecy
These may be apocryphal, or not. The 16th Karmapa made a point of visiting the Hopi Nation during his visit in 1974, as did the 17th Karmapa, in 2008. You might get a more scholarly response if you ask your question on the sister site, dharmawheel.com