Cage:

The Retopology Wiki page does say that it can do this:

"Projecting an existing mesh onto the object."

This sounds like what you are thinking about. Sadly, the Wiki Manual does not tell how to do this.

LMK

Edit:

How about this Blender function - Shrink-Wrap:

http://wiki.blender.org/index.php/Doc:Manual/Modifiers/Deform/Shrinkwrap

LMK

Probably edited for spelling, grammer, punctuation, or typos.