This plug-in is included in the Rhino 5.0 WIP. Command name is BlockEdit in V5.
Summary: In-place block editor prototyped in a plug-in.
Unzip the two files files from the zip archive and load (drag and drop into Rhino 4.0) both the plug-in (.rhp file) and the toolbar(.tb) file. To install the plug-in, use Rhino's PlugInManager command. To load the toolbar, use Rhino's Toolbar command.
This tool does not work on block instances which have been scaled non-uniformly.