I had the same problem and was able to get the 64 bit version of EsriRegAsm to succeed by removing the reference to ESRI.ArcGIS.ADF, and adding a reference to ESRI.ArcGIS.ADF.Local, and recompiling.
However, when I go into arcmap and run the tool, then right click to share as a geoprocessing package, it fails.
I have both arcgis desktop and server installed on the same machine.
I'm getting an error 00151.