The way I read this you do this from a terminal ?
okular somefile &
I would think doing this would work (assuming sh type shell):
okular somefile > dust 2>&1
but I gave it a shot and some messages bleed through while some are redirected. So I would guess okular may spawn some new processes. I'll play around a bit and see if I can find a workaround and post if I find something. I find this behavior annoying also.