I've tried industrial ammonia, kroil, pb blaster, heat, cold, Freeze Off, but most are no go. You might loosen the seat pinch bolt, keep adding the penetrant of choice and ride it for a season. It may loosen up over time with ride vibration. Since it can only sink an inch from it's current position, you won't be traumatized when and if it moves.
If nothing happens, then corrective surgery with a sawzall and a wide tooth blade.