Has anyone who's NOT a pro painter ever tried it...

Maybe? I tried two digital versions (using Photoshop) several years ago, one based on Bob Ross techniques, and another based on a Dana Jester guest episode painting. The latter came out awesome; the first was silly (though it did kind of look like a Bob Ross painting).

In real life, using real oils, I remember a kid bringing in his Bob Ross painting for show and tell (8th grade in the USA), and everyone was floored. It was really good. I remember him saying it was his first painting and that he did exactly what Bob Ross told him to do.

I've no doubt it works. Oils are expensive though. So someday...

btw (i feel like a spammer; i'm not, I'm just an excited fan), there is an official Bob Ross youtube channel that has EVERY (season 1 included) episode up, in its entirety, and legal. Very cool stuff.