Let's consider two more unusual thngs.
1. Does it have a servo? If it does, it could be brake fluid burning due to a leak in the servo unit allowing the fluid to leak into the induction (vacuum) area. .
2. Is there any chance it could be steam? I know it sounds unlikely and I'm sure you would have thought of this, but thought I would mention it. A small crack in the head in the exhaust tract area could let a small amount of coolant into the hot exhaust area.