Thinking about the mechanicals of this a little more, I've been considering servos and steppers. I don't need speed or precision, really. But I do need to be able to move in either direction, which I now understand to be complicated with servos. So I considered steppers, which at least can move in either direction.
A particular challenge of this application, though, is once the rotation has happened I'd like it to be locked in place to resist rotational forces from wind and chop, and I'd rather that not draw power, making even a stepper with holding power difficult. So I'm thinking that I need some type of normally locked mechanism. Something like a ratchet, but I'd still like to be able to move either direction, without full rotation.
Perhaps something like a stepper motor and a pin that can be dropped into the rotation mechanism and retracted using a solenoid? A little googling suggests that I might be looking for a "power-off brake" but these all look very expensive.
Any ideas? Honestly the holding tension doesn't have to be absolute, just enough that the motor can be turned off when in position and have some expectation of holding in non-extreme conditions. Perhaps the initial pressure can always be in place and just have the stepper push through it to move the thing? That does seem like it would draw extra power, though.