Brady had a number of wagons that were mobile darkrooms for making and processing plates.
He had a lot of people working for him, just as Ansel Adams had many many assistants and associates. Al Weber, who teaches an excellent workshop with David Vestal at the Photograper's Formulary is on such associate. He speaks fondly of his associaton with Ansel Adams. I had a chance to sit with him looking out at the Mission Mountains on our last visit there which coincided as my wife took photos of us and our own Jan Pieterzak (sorry for the spelling Jan) unloaded their van.
So, yes, this practice is common, but they all hearken back to their mentor. Brady was a mentor of these people and did travel with them.
We do not use this apprentice or mentoring system much anymore.