I think modeling clothes after posing will give twice more work then modeling them on T pose. And will be more difficult for you to determine how they will behave. If it was my model, I would model the clothes making my model live, making some curves on top of it, where natural seams on clothes go and using loft/boundary to generate some nurbs surfaces, making the clothes geometry.
After rigging and posing, I would use the sculpt geometry tool to make folds more naturaly, and when I got the results I wanted, I'd convert those nurbs to polygons, getting the final clothes.
I don't know if you like the idea, but I think doing clothes is much easier using nurbs than polys. Maybe this idea could be used as inspiration for something different, though