Dynamo hub interchangeable?
 

I saw a Shimano hub at a yard sale that has a disc brake rotor on it. Is it possible to take the disc rotor off and use it on a fork with cantilever brakes? I know nothing about disc brake hub sizing vs standard hubs.

Should be fine.

The hub will have asymmetric flanges because of the disc mounts. This means the wheel will have to be "dished", similar to a rear wheel.

Depends on the rim, does it have brake tracks? A few don't.
@sweeks >> Yes the wheel IS dished, so it will be weaker using a rim brake. But it can NOT be changed.

OP said it was a hub, not a wheel, so he can (hopefully!) get a rim with brake surfaces.
Not sure I understand why a dished wheel is weaker with a rim brake, assuming it's properly built.

EDIT: Heh... I notice we both have exactly the same number of posts at this time: 1,749. :D

In your case it wouldn't be too bad a kluge but why not just get the proper hub if you don't already have it? Shimano 3N72 dynamo hubs are not that expensive, although I just looked it up on Amazon and it is a lot more than I paid a few years ago.