I am guessing 24VDC elements.

Must be 50 ways to do it.

It could operate a giant relay as long as the hysteresis was set considerably higher than normal.  I am anti-relay for most things.
Maybe a giant self contained IGBT.  Or 2.

I figure this will need to be several heating elements.  How many?  What rating each?

My circuit is single stage.  With 800AH at 24V, might consider something 3 stage, or at least do manual equalization with my circuit.

There are some current capacity boosting hacks that can be done to a popular multi-stage controller you did not mention.  It would be an option.  
Not sure I want to throw them out for general public viewing.

If the alternator is efficient then it will stall the prop as a battery charger and your power out will be way down on what you saw before. If the alternator is not stiff enough to hold the thing in stall then you will still see much less power into the batteries yet you will see a great increase in stator heating so you may need to rethink the furling point.

If the alternator is stiff enough to stall the blades then if you want lots of heat then I would add one heater as a series resistance in the dc line to the battery after the rectifier. This will be a really awkward resistance and you may struggle with a commercial heater. It will let the mill run as previously away from stall and your total power out ( heat plus battery input) should be similar to previously. You will see an increase in battery charging capability compared to running stalled. You will still need to watch stator current limits but you will get more battery power and the additional heat for a given stator heating.

Having got part of the heat directly you then will still need to divert excess power as heat when the batteries are charged.

" I am sure most have seen the 400 A rated switching relays advertised out there for about $50.00 ea. Ghurd, what about using your controller to trigger a relay such as this which would switch the DC output from the batteries to the heating elements at selected fully charged (high) battery voltage ? "

That is a pretty deadly form of control and will take some handling. You change from battery load with stall to resistive load with or without it and things will either stall out or get away.

Normal diversion loads work with the battery connected and you just divert excess current to maintain battery volts. I don't think much of relays to do this at the sort of current you are expecting. It is really far better done with some form of pwm control either intentional or brought about by a small differential switching load using battery voltage rise due to surface charge to do the proportioning.

To do what you propose with switching away from the batteries to a heater needs a fairly clever controller or lots of careful selection of resistors and a fair bit of luck and you will probably not find relays up to the job for long although you might get away with a series diode and divert before the diode so that you still have the battery to catch things when the heater looses control but you will be in stall mode and heat production may be rather disappointing.

With a correctly chosen series line heater and a pwm dump you get a decent battery charging scheme with a fairly effective and simple heater control, not ideal from either point of view but a good compromise.

Just a few thoughts

Once it starts running at 15MPH, does it keep running at say 12MPH?