One support for that Castaneda had an ascetic vein is given Barbara Meyerhoff. You'll find this qoute in my chapter about Mars in Scorpio, which CC had.
Richard De Mille interview (pp 352-53):
"Barbara M: Did you know he lived in San Diego?
RdeM: He did?
Barbara M: For a while in a basement room.
RdeM: He told you this?
Barbara M: Yes. In a friend‘s house. I thought it was rather a long commute to UCLA, but those things never seemed to bother him.
RdeM: Not when he could go back to Mexiko in the blink of an eye.
Barbara M: Maybe that‘s it. Anyway, he said he gradually emptied the room out. First he got rid of the bed. Then he got rid of the books. Until there was nothing left but him and the typewriter./.../ he overcame it by giving up all his normal habits. He gave up food. He gave up sleeping. Honing himself, so to speak. And I thought it was all rather shocking. But he was training himself in ascetism. He was a profound ascetic streak. If I had to give you a psychological intepretation, I might say he´s a tragically isolated man, struggling for discipline, dominated by his will, animus-possesed, anima-terrified, seeking impeccability, seeking passion with control. Willful ascetism. He was merciless with himself. Though he indulged himself by not giving in to the system, at the same time he sternly made himself give up personal relationships. Maybe he really wanted to give them up, but I felt it was a struggle. Made himself give up that little boy. Made himself be there in the library every single day, perfectly dressed in the dark suit, with the briefcase, everything in order, a narrow, determined quality. I suspect it‘s getting fiercer and deeper as he gets older."
"He was merciless with himself," ....