The people that really need that functionality go with a 580EX mounted on the camera's hotshoe and use itforAF-assist as well as a master forfiring off-camera strobes (via built-in wireless or Radiopopper PXs). It's costly, yes, but not too many people are taking pictures with off-camera strobes when it's too dark to focus...else there would likely be a more affordable solution like you're wanting. There might be a product that fills your needs, but if there is, I haven't heard of it.