Hi Peter, nice seeing you around.
I guess you're right. Elvis always seemed to have a nervous thing with his hands. Photos from the July '69 press conference show him always kind of bending his fingers and just playing aournd with his hands, pretty sure a way to handle his nerves. And there are many other moments like that in his life as you pointed out. So sure I guess just to hold at his guitar pick was kind of a nervous thing, just like many of us (or at least myself) will start drumming in the desk with the pen or something at stress points as you pointed out.
It is well reported that Elvis was pretty nervous for the Aloha show, many people said he seemed stiff or unnatural in that show, well I'm sure there where reasons for that, the need of right timing because of precise airing time, the I can't make a mistake on this one as the whole world is watching this live... man I couldn't do it.