Hmm...

How do you mean, your front hub is "special"? If it's an emotional attachment, put it under glass and get a dynohub. If it's a mechanical compatibility issue of some kind, like it's the only hub that fits in that fork, then it'll be difficult at best.

I suppose that you could get a rear hub with a disc rotor and adapt a bottle generator to run against it. It wouldn't drag as much as if it were using the tire sidewall because the leverage will be more in your favor. But then again, it'll be turning at a substantially lower RPM than it's designed for, so it won't be putting out nearly as much power as you'll expect. It might not even give enough to power a headlight.

There are, or have been, crank-driven generators. They only work when you pedal, but they don't require replacing either wheel hub.

*edit* Well, I thought that I saw something about such generators at one time. I probably got confused about bottom bracket-mounted generators, which are a lot like bottle types except for where they're mounted.