A friend of mine did one for a small 6ft machine at 50ft, it worked very well and I suspect you could scale it up a bit. Not sure how big a machine I would want to try it on. My friend added ship type rigging on the top tower section to make it rigid enough to hold the turbine weight when horizontal.
The counterbalance for that small machine was about 1/2 a ton but the lower section was only about 10ft rather than 20. Your pivots need to be very strong and again ship type rigging was used to prevent the pivot shaft bending.
When working it is very tame, the thing is self balancing and the only force is the weight on the legs supporting the pivots. Guys are used when up in the usual way.
PLEASE take note of what Scoraigwind said about the machine sliding off the yaw pivot, build it some other way, if it does slide off you will loose the balance condition and end up with a rocket launcher ( we got seriously close to doing it ).
Flux