I think Cary's statement about Siegried Behrend being a proponent of an alternative CG position needs to be corrected (at least partially): if you do another search on youtube for "Siegried Behrend: Die Gitarre", you can see why:
In this video, after performing a nice flamenco-like piece, he explains (in German) some basics about the guitar sitting position, and then goes on about guitar history, etc.
Starting from about 2:50, he says (my translation): "If you see me playing like that occasionally [with guitar on right leg] in my recitals, [...] that's simply because I often love to eat quite a lot, which is why I am a bit fat [sic
], making it very strenuous to sit like that all night. But this [putting guitar on left leg] is the only correct way to be sitting when you play the guitar."
Make of that what you will. At least he's remarkably frank for a TV show in the sixties or so. But it seems he didn't advocate an alternative CG position. At least not officially
(BTW his left-leg posture seems similar to Pepe Romero's.)