What I was actually thinking of would be similar to the "bottle" type dynamo, only DIY and perhaps using a cheap DC motor. As an aside, most rotary electric motors can be used to generate electricity when the motor shaft is spun by an external force, though they are often significantly less efficient than purpose-built generators.
Also, if you decide on using a generator, commercial or DIY, I don't see how you'll be able to avoid runing at least some wires on your bike. Slip rings
can help in places where you have rotation, and might be included in a commercially available design. If you do go the DIY route, here is a link
to a couple that are convivial with a limited budget and could fit this application.