I use a PL60 to shunt regulate a mill that peaks at just over 1kW @ 48V, at 20 - odd amps its well within its max rating though.
My mill uses pretty much the same furling method as Hugh's design; from my observations, I'd agree with your presumption that max output occurs just before furling. Typically, a sudden increase in wind speed (or change in direction when the mill's partly furled) will result in a spike lasting maybe a couple of seconds, I would rarely see more than 750W sustained.
Having said that, a resistive shunt load will draw more current as the battery voltage increases. If your battery voltage starts to run away because you can't dump the extra 10A, then you could find the shunt load pulling more than 30A, possibly resulting in a fried controller. Not worth the risk in my opinion.
I've never seen the expansion board but I've seen the specs for it somewhere. If you end up using it please let us know how it goes, when I add a bigger mill to my system I'll probably need one!
If in doubt, give Plasmatronics a phone call, they're pretty helpful in my experience.
Paul.
Off grid 5 months and still married.