I got it working now, I used the guess_legacy_field_names() to check what was the correct field for the rule '001', and it worked with '_id' so I just added it based on the sip.metadata['recid'] while making the record (and before producing the marcxml).
A post-upload hook would take care of updating record ID <-> SIP relationship in this approach. Kind of like "bst_send_email.py" takes care to email the original submitter of a document only after the record is fully ingested. I gave preference to the other approach, seemed less complex and with less post work to do. Thank you for all the help and suggestions, Pedro
