Have not used it myself but I think you have the answers in the IMassProperty interface. See link
And yes... it looks the be configuration dependent. So you may set values for each configuration.
Mass is just a property that you either link to calculated value or fill with any other value. So for example if you need property called "mass" to contain value then search for APi eamples (https://forum.solidworks.com/message/441679#441679 )about writing custom properties.
Mass can also be set to a value for the part or assembly. By doing that you override the calculated. Which means override value will be used when calculating the mass where the part/assembly is used.
Override the mass property is not equal to overwrite a custom property.
that was exactly what I was looking for.
I don't need to create a property, I want to force the actual mass of the part.