I need this too, but haven't figured out why the standard WGM will not work.
The WGM should modify the toolbar through the CUI of AutoCad, so that should be common.
The file format of a P&ID file is standard AutoCad DWG format.
The only difference between AutoCad and AutoCad P&ID (and Mechanical, Electrical, etc) is the libraries that are included with the package, from wjaty I can tell.
Why is PTC saying that the only support the base AutoCad and not the packages in the WGM?