Sorry, I should have been clearer on my gear weights. I choose running shorts, which are only about 3oz. I don't need the pockets or length. My Capilene long johns are about 4-5oz, like yours. Windpants or rain chaps I would use also come out to the 4-6oz range. The GoLite Reed or Whim pants, Montane Featherlite pants, and Equinox Full Moon chaps, GG SpinnChaps, & Oware chaps are the specific products I have in mind.
When and where will you be on the AT? For much of the standard hiking season on most of the trail, I think shorts will see you through just fine while moving. For something I use so rarely, I just want them to be small, light, and fairly cheap. The options I listed above fit the bill for me, and are also durable enough for my needs.
If you haven't seen me plug the site already, www.whiteblaze.net is easily the top AT information resource. Lots of great folks in the forums there that I'm sure could also help you with the planning.
Edit: Just have to agree with Dale that comparable items are pretty easily available in thrift stores. Running shorts/swimming trunks, windpants, and rainpants, especially. And at those prices, you don't feel too bad about making modifications, trimming, etc.