perhaps a more complex version of the variable attenuator method mentioned previously with the "spring" that has a contact moved down it...
This is an RF variable attenuator (what I am more familiar with) There are similar ones for AC currents and DC currents that perform similar functions for power. I am just not 100% sure where to find them.http://www.jfwindustries.com/catalog/Single_Rotary_Attenuators_50_Ohm-57-1.html
this one again is for RF, but I am sure it can give ideas as to a method to manage the power comming from the mill that you can adjust to help control stalls and such.
Perhaps if you wish to get more complex you can have a "controlled" attenuator with a method of monitoring RPMs of the turbine, where the attenuation is adjusted to the needs of the mill at the current RPM to make the power output more "efficient" to your needs/equipment limitations at that RPM. (This will require you to know your equipment limitations and have ideas on the limits of your rotor, battery charging, yada, yada, yada...... you get the idea... I am not an expert by any means, but it is Sine wave manipulation in either instance, which is what we are trying to do in both circumstances. (RF and AC)