unfortunatly the equal relationship established in the sketch is not transfered to the sync part.
dont forget that fundamentally sync disposes of sketches and is there after driven soley by face sets.
here is only method to drive a square.: http://screencast.com/t/zyiNeNTyP
But in this you have to have locked (driving) variables and thus do not get to drag the face.
or You simply use the polygon function within sketch, draw a 4 sided polygon and extrude this one to Your block.
Using a polygon creates a 3D relation polygon, so that all 4 sides allways keep equal.
Same as having a hexagon profile
But the more important question would be:
"is there a way to create this polygon relation 3D manually?"
Or is this (using polygon sketch feature) the only way to success?