This thread is suffering from a lack of typing.

I think some of us are talking about slightly different things. Here is SolidWorks behavior as I understand it:
There are two types of custom properties: "document" and "configuration specific". The "document" custom properties are shown on the "Custom" tab, and the "configuration specific" properties are shown on the "Configuration Specific" tab. As a side note, these are stored differently in the file. "Document" properties can be accessed outside SolidWorks just like custom properties of Excel files, etc. "Configuration specific" properties can only be accessed by SolidWorks (or SW Explorer, or swDocumentMgr.dll).
When you link to a custom property, you specify the name of that property. SW first looks to see if a configuration-specific property with that name exists. If so, its value will be used (even if blank). If a configuration-specific property does not exist, SW will check for a "document" custom property of that name and use its value if it exists. This behavior is the same for drawing notes, BOMs, etc.
So, to sum up, any custom property that should be "Inherited" (have the same value for all/multiple configurations) should be specified as a "document" level property. Any links to that property name will retrieve the "document" level property value - regardless of the configuration referenced - unless a "configuration specific" property with the same name exists. If that configuration specific property name exists (even if the value is blank) it will override the "document" level property.
Does any of that make sense?
That said, there are a myriad of custom property related macros out there. I'm sure there's probably one that will copy configuration-specific custom properties from one config to another. Have you checked out the resource pages in the FAQ area?