Can a co-processing pathway — where biomass is gasified and then combined with additional RFNBO-H₂ — qualify as an RFNBO under ISCC EU? 

Yes. EU DR 2023/1185 Annex A.1 explicitly permits co-processing of biomass with RFNBO-H₂. The RFNBO output share is calculated according to the RFNBO-H₂ energy input with respect to the total energy of all inputs. The remaining share might qualify as biofuel — provided you hold ISCC EU certification covering renewable electricity, mass-balance traceability, chain-of-custody and GHG accounting per ISCC EU 205-1 for RFNBO and ISCC EU 205 for biofuels.