The VI Toolkit contains cmdlets (New-CustomField, Set-CustomField, Remove-CustomField) to manipulate the custom attributes.
Did you try using those ?
You can't write to the SDK properties like that.
You have to use the SDK methods to change settings.
But like I said, there is in fact no need to use the SDK, you can use the VITK cmdlets.
Worked like a charm. Thanks for your input.