catcher holding ball in hand tags BR after dropped 3rd strike, holds tag for a brief second and then ball drops out of his hand.

HTBT.

it's a hypothetical question.
so BR is not out the instant of the tag? ball must be controlled for a while after?

The ball must be controled before,during and after the tag. Now if the umpire rules the ball came loose because of the tag then he is safe. If, as in the Veratek play last year, there is a seperate action after the tag that causes the ball to come loose the the runner is out.