Enhancement to edit customer line on reversal accounting doc. (FI) resulting from transaction VF11

I need an enhancement to clear fields XREF1, XREF2, XREF3 and CESSION_KZ on the customer line of the reversal accounting document generated in FI from transaction VF11. Ideally this would also work on accounting docs generated from credit memos of type G2 created with transaction VF01.

Things tried so far:

BTEs: BTE 1120 doesn't allow modification of field CESSION_KZ.

BADIS: the BADIs for billing (BADI_SD_BILLING ...) don't work because 1) they're marked as internal to SAP and can't be implemented, and 2) they're not called in the case of a cancellation.

USER EXITS: EXIT_SAPLV60B_002 is not called during cancellation.

Please help. :)