Well, thing is, if you assign a turret to a subsystem, even a rotating one, it will work perfectly well, (except for beam, since the fire-point does move once it starts firing).
If you destroy the subsystem that is classed as the objects parent, it will also destroy the turret (though, admittedly, there still a slight bug of it not destroying the fire-point).
As far as I'm aware, this
should work with non-turret systems, so you should be able to 'stack' subsystems etc.
It doesn't help with assigning two parents to a single system, but it makes for the possibility of some great non-tagettable greebles
