Using C++, I get the mass properties using:

    SldWorks::IModelDocExtensionPtr swDocExt;

    swModel->get_Extension(&swDocExt);

    long swStat;

    _variant_t mprops = swDocExt->GetMassProperties(0, &swStat);

The mass properties are returned in metric units (as the documentation says).  I want to convert mass to what the user sees when the Mass Properties button is pushed.  There must be some "convert-to-user-units" API, but I have not found it.  I don't want to write a general converter.