There is an interesting article on idioms used in the Bible in the Dictionary of
Biblical Imagery, pp. 326-328. It lists several uses of "dust" (BDB 779) in mourning:
sit in dust, Isa. 47:1
roll in dust, Micah 1:10
bowed down to the dust, Ps. 44:25; 119:25
dust (ashes) on the head, 2 Sam. 1:2; Neh. 9:1; Esther 4:1; Job 2:12; Isa. 58:5; Jer. 6:2