There are little plastic thermal pads that are avaliable that are normally used for putting between MOSFETS and the heatsinks cooling them, as far as I am aware they are non conductive (can someone confirm this?).
If they are you could put one on the core (only need to do the very centre between all the surface mount capacitors and resistors and then put a heatsink on top of it (may need to mill around the edges to avoid the surface mount components). Then simply bolt the heatsink through onto the waterblock, without actually bolting onto the card itself (just go straight through the holes). That way the heatsink is applying pressure to the middle of the core, pushing it into the waterblock while simultaneously cooling the back of the board.
