Well to be perfectly honest plain water will cut copper given enough time at any flowrate and pressure above zero, it's called erosion(not trying to be smart alec just making a point). Given this fact unless your particles are smaller than a water molecule then they will accelerate the process, regardless of density of the material. The trouble is how much erosion will occur over a given timeframe in the typical WC'ing setup. Granted cpu, gpu blocks get changed when ever their is an upgrade so if the nano fluild takes ten years to show signs of wearing the block I think it would be safe enough on the blocks.

My concern isn't the blocks but the bearings in the pumps, because these small particles will get in between the impeller shaft and the bearing. If your particles are like bucky balls the bearings would actually last longer. Aleternitively if they are shaped like cubic zirconia crystals they will eat the bearings up in no time flat.