8va is a score indication - so it wouldn't need a nudge from a conductor would it - Like there's no direction to switch to pizzicato or back to arco - Conductors are looking after timing, volume attack and all those things that cannot be done by the score alone.
I suppose that if a piece was written that needed the musicians to suddenly switch octaves (and I can't think of one), then the conductor would simply devise one, make sure the musicians knew it, and do it.