Bulkloading a Custom Assembly varies slightly from bulkloading a symbol definition:
The Custom Assembly ProgID (in this case, assembly .dll name with custom assembly class name and its namespace), defined in the Definition field of a bulkloaded spreadsheet, indicates a Custom Assembly.
When this field is present in the spreadsheet for items such as equipment, footings, equipment foundations, etc., it is expected that the Definition field should be completed with the name of a Custom Assembly. The SymbolDefinition is left blank (that is, this field is ignored).