The 3 known debit schemas (pain.008.003.02, pain.008.002.02, pain.008.001.02) all support the InstructionId field in the PaymentInformation next to the EndToEndId.
In sepa_king this field can be set using the instruction key when adding a transaction to a sepa document and it works for credits but not for debits. For debits it is even specifically mentioned in the README.

Is there a specific reason for this to be missing? Otherwise I'll prepare a pull request to add this, because we actually need this functionality.