Help me refind this 3d go variant - pieces can be stacked in a pyramid fashion
It sounds like it might be Margo, except that Margo is played on a 7X7 grid, not 9X9. Per the linked website by Cameron Browne:


  Two players, White and Black, each have at least enough marbles of
  their colour to cover the board.
  
  Start: The board is initially empty. White places the first piece at
  any board point, then Black may elect to swap colours in lieu of
  making the next move (swap rule).
  
  Play: Players then take turns placing a piece of their colour either:
             1) at an empty board point, or 
             2) stacked on top of four existing pieces (of any colour).
  
  Players may not pass.


The capture rules sound like what you describe, and I think the "pinched" stones at the bottom of "pyramids" are what the rules refer to as


  Pinned Pieces: When a group is captured, any of its pieces pinned by higher-level enemy pieces remain in place. For example, move c captures the following White group but leaves two pinned pieces.