I once operated an FCC that sent coker naphtha to the riser. Can't share specifics plus it has been over 10 yrs. High temperature in unit and acid sites handles the diolefins adequately. Injection on the unit was at the bottom of the riser instead of lift steam. Had data that naphtha helped dry gas yields slightly as it worked to quench hot catalyst from regen before contacting feed. Remember that a higher percent of sulfur in the feed passed through to products than virgin feed ( vs. producing H2S ) due to the fact that material was already thermally -----ed.