I hypothesize that in an extended experiment we could actually brainwash ourselves to the point of holding some radically different views.

This is the first of Lifton's eight criteria for thought reform -- one is systematically exposed only to one side of evidence, and isolated from the other side of evidence.

To make this brainwashing more effcient, there are additional techniques:

Live among people who will make you feel ashamed for not believing in X. These people should love you as a person, but hate any non-X-ness. Expose your doubts about X in front of the group -- it will help them understand and modify your mind processes.

Develop a group-specific jargon -- if you make your pro-X arguments using words that an outsider does not understand, then the outsider cannot refute these arguments.

If you ever experience or remember something that disagrees with X, be a good Bayesian and remember that with probability at least epsilon, your experience or memory is wrong. On the other hand, assing prior probability 1 to X. This way, whatever evidence you have, it is perfectly rational to believe in X.