"Which poser version are you using?  If you are using the Pro series with gamma correction, and forget to set the Gc of the displacement map to 1, you will get a distortion of the displacement thus causing pokethrough. "

I'll check that.

As far as this suggestion:  "Seems to me the easiest thing would be to assign a new material to those polygons in the group editor.  Select the part and in the group editor and assign a new material with a name such as "skin2".  Then go to the mat room and select the base and all of the nodes of the original skin and copy them.  Then you can paste them into the new skin2 mat and lower the displacement.  Once the material is created you can then select other parts of the body if needed and apply the skin2 mat with the drop-down menu.  Of course you don't need to change the mat of the entire part, you can use the "+/-" selection modes to remove any polygons that aren't under the shirt mesh."  That's what I did.  It creates horrible seams, but I think in retrospect those are the areas between the displacement and the non displacement textures.

 

I'm also going to try the mask.  I've had good results from transparency maps.  So maybe that will be as easy and effective.