Work tables are only usable when accessed from the associated specific Sage printing function.  There are zero exceptions to this.

You are correct. When printing is started the programming behind it creates a works tables and gives it a time stamp. This is the file that is used for the report and not the basic works file so no it is not possible to use another works file because it is empty and there is no way to populate it.

The best thing to do is convert the Packing List to use the SO_Invoice table instead.  Once you convert the report to a none work file, you can run it outside of Sage.