The basic concept of the derailleur doesn't seem too complex, however, I am worried about what may be important dimensions and way may not.
From what I gathered, the derailleur is really just a spring loaded four bar linkage with an attached tensioning mechanism.
So, the parallelogram formed by the four bar linkage skews to the side to line the sprocket up over the various gear ratios, and it drags the chain along.
The wire is used to keep a tension spring within that linkage stretched, to hold the derailleur in place. By increasing or decreasing the tension in that wire, like you mentioned, Ian, you move the derailleur.
A piston or a servo, however, only has two definite, reliable positions. There are ways of stopping each midstroke or midturn, but it'd a bit inprecise, and may result in a half-shift, or missed shift.
So, the key is a simple mechanical index that gives exactly the right number of positions. We can build one of these using the small electric motors included in the kit.
We can't buy a bicycle derailleur, I think, because it designed for a different sort of chain and sprocket than we'd be using in all likelihood. Plus, since it's not a legal kit part, we'd have to design our own anyway