Would it be possible to plot on an object with a soft step in it? Or an uneven surface, provided the surface was smooth with no angled corners or overly large steps?

Like so.

http://www.flickr.com/photos/edermiller/4978156152/ Here is a link to an example illustration. Apparently I cant upload a file.

oo. Tricky.

I don't know how well that would work. If the curve were a bit smoother it wouldn't be any problem. Going around the axis of symmetry, you could go all the way around at any point and it would be okay. But drawing a continuous line from (close to) one pole to (close to) the other, you'd probably get stuck on the discontinuity.