Ive done some searching and it appears that for SW2010, there was a solution.  But the "Cavity" feature does not seem to exist any longer in 2012...

I currently have an assembly of a die.  There is a part with some complex geometry at an awkward angle that I would like to "subtract" from the part that it interferes with.  I would then use the "move face" command to add some clearance to the part.  The parts dont really referance each other in any way, so using the "Combine" tool coupled with an "insert part" within part mode, wont really work because I cant locate the parts to each other.  I really need to just pull the geometry from the assembly.

I tried doing an "Edit in context" and making surfaces from the part I wanted to subrtract...but this is a HUGE pain.  Is there a better way?