I would totally forget the idea of using universal joints. Not only they will add considerable weight, but a lot of complexity to get them to operate right and they are quite sensitive. Big gyros behave differently (that's why 2 blades work fine, but on models they are tricky to trim), not because different laws apply to them, but because those laws depend on the size of the object.
And, after all, autogiros are about simplicity. I also believe that the simplest solution is often the best one.
The engine starter is the best aproach IMHO. If you can find a good combination of motor and reduction, which is the "difficult" part, and mount it right on the rotor (the pic shows it very clearly). On top of that, a brushed motor and gears won't cost you much.
I also have a very promising idea in mind for my ECDC (great flyer, by the way!!!
), but I want to try if first. It's very simple and reliable, or so it seems in theory. In fact, probably someone has already tried it.
BTW, have you tried Aerobalsa blades for the model? I'm quite tired of making my own blades...