What if you put up a 40mm steel tube, like the stuff they use for handrails cos it's pretty cheap. It needs to be the height that the tower will end up, I suppose.
Put a length of PVC sewer pipe over the bottom bit, up to the guy height. Run your guylines up inside the PVC pipe and pull them through holes drilled in the steel tube at the right height for the guys. The bottom of the guys would be anchored to eyes set into the tower base.

I suppose you'd need to oval the holes and have smooth edges so as not to damage the cable. Or maybe some kind of turnbuckle to help with the fairly steep angle. I've seen cables with these sorts of angles before in rigging, so should be do-able.

Then tension the guys until the steel tube is perfectly straight. Drop on the top section of PVC pipe so it's level with the top of the steel tube and fill the sewer pipe with concrete. If you made the 'crete with fairly fine gravel and put plenty of PVA in it it should fill from the bottom pretty well with a bit of mallet tapping.

Once the concrete goes off you should be able to bolt on a reasonably big turbine. The tensioned guys and steel tube should act as pretty good rebar for the bottom of the tower, where the forces would be strongest in a heavy wind. The top bit wouldn't be as strong, as it's only got the steel tube for re-enforcement but should still be OK. And the plastic pipe would protect the concrete from the weather.

I drew a diagram to show what I mean, but it won't post for some reason. Think my browser is acting silly.

T