>I think that I'll probably start with just eliminating them (seam edges on closed smooth surfaces), and then if anyone ends up needing them I'll look into adding an option later.

Works for me, thanks!

>Unfortunately the information controlling that is likely to be private just to illustrator - Illustrator has become rather closed with its internal file format structure over the years, the only documented AI version is from AI v8 .

My reference was just for people that use Illustrator CS5+ and want to edit line widths. Although Astute Graphics must have access to current internal file format structure as they are a 3rd party developer. Anyways, I'm not requesting this as an export option so no worries. :)

>I don't think that PDF has a method for tapered line widths, so probably when generating a PDF file Illustrator converts tapered strokes into a closed and filled outline path rather than something that actually has a line stroke on it in the PDF.

Yes, that's correct. I understand that users may be using Corel, Inkscape etc. so the export needs to be compatible to more than just Adobe.
I'm staying on the ground with CS6 and will not be moving to the clouds.