I've worked with mobile lifts before, and although they can't be raised unless the engine (and hydraulic pump) is running, the ones I've worked with can be lowered even if the engine is off.
Perhaps you could constuct something similar to an anemometer, and when a certain windspeed is reached (10 MPH over furling?) an electronic switch is tripped and a battery actuates a solenoid that bleeds the hydraulic fluid through a drain valve to the supply tank, lowering the boom.
Even if this idea is sound, there may be a better way to "auto-lower" than the switch method I've suggested.
If your heart is set on getting a lift as a portable tower, it's possible nobody will be able to talk you out of it. It would be handy for towing your windmill to demonstrations, but if the wind exceeded 50 MPH, I can't imagine there being any people around.
It sounds like you want to set it up and leave it unattended, if so, I agree that you should add guy wires, perhaps anchored by tanks that are filled with water after placement. If a big storm comes through before you can get to it (to lower it) you should set up a fence far enough around so that if it does pull up the anchors and fall over, nobody is near. Even with a fence and warning signs, stupid people could be inside the fence and injured. Even if you are not legally liable because of your precautions, it can still be very expensive to go to court and prove it.
"If you don't have anything good to say about anyone...come sit by me" -Alice Roosevelt
