Turns out the doc about rock guitarists isn't the film's only cinematic tie-in. The Band's official website mentions not only Easy Rider but...
The Weight became a signpost of the time. Featured in the film Easy Rider (but not on the soundtrack for contractual reasons), it was partially inspired by the work of Spanish filmmaker Luis Bunuel.
"He did so many films on the impossibility of sainthood," says Robertson about Bunuel, "people trying to be good in Viridiana and Nazarin, people trying to do this thing. In `The Weight' it's the same thing. People like Bunuel would make films that had these religious connotations to them but it wasn't necessarily a religous meaning. In Bunuel there were these people trying to be good and it's impossible to be good."
"In The Weight, it was this very simple thing. Someone says, `Listen, will you me this favour? When you get there will you say hello to somebody or will you pick up one of these for me? Oh, you're going to Nazareth, that's where the Martin guitar factory is. Do me a favour when you're there.' This is what it's all about. So the guy goes and one thing leads to another and it's like, `Holy shit, what has this turned into? I've only come here to say "Hello" for somebody and I've got myself in this incredible predicament.' It was very Bunuelish to me at the time."