I'm not sure I'm getting this business of the thumb plucking up - I've presumed you mean away from the belly - but have I got this wrong? If it is as I've been thinking you meant (ie plucking the string upwards - away from the belly) doesn't this lead to much slapping of strings onto the fingerboard/belly if plucked with any great vigour?
Indeed in a slight angle away from the corpus (a guitar has a slender waist). I think I have seen many lutenists and guitarists doing that. This is probably one reason why most baroque guitarists have no clear bass when playing the (octave strung) fourth course.
best, Lex
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