We could "mount" any wind vane on the back. The mounting would be simple, but the vane would be horrible at steering the boat. Our boat regularly sees speeds over 10 knots and wind vanes just aren't made for those kinds of speeds. That's the reason you don't see wind vanes on Around the World race boats. Not because they are "race" boats, but because they are very fast boats.

I debated this issue with myself at length before crossing the Atlantic. We ended up with just an autopilot, but because that was our sole source of steering assistance we didn't buy the cheapest autopilot we could find at West Marine, instead we spent a considerable amount and bought what I think is the best autopilot available (NKE w/ an L&S ram). Nothing is undersized, or cheaply manufactured. Corners were not cut. The autopilot is built to be reliable. And to one up the author of the wind vane vs. autopilot article our autopilot will steer a compass course, or to apparent wind (like a wind vane) or to true wind, or to a waypoint (never use this option, but we could).

Actually, it wasn't our sole source. We had the original Raymarine belowdecks pilot as a backup.

As it was the autopilot steered almost exactly 99% of the 3300nm crossing, even when we were regularly hitting 10-12 knots.

For a boat like ours, the only value a wind vane can provide is a poor backup in the case of total autopilot failure. That could happen, and we may get a wind vane (Hydrovane or Wind Pilot) not because I think we'll use or that it will work well, but as a belt and suspenders approach, given the Hydrovane and Wind Pilot can also provide backup to the entire steering system/rudder.

You may call a J/120 a race boat, because it is a fast boat (for a cruising boat). I would call it a cruising boat because it has all of the conveniences of our old Valiant 32 (clearly a cruising boat). Zero is given up in terms of comforts, but we travel on average 2 knots faster than before.