What I am thinking of is a program which allows you to define and reposition coloured lines which show the position of body parts in a turnaround.
If you replace "coloured lines" with "bones", then you're pretty much describing how AS works already, with a typical image-per-limb setup. Especially when you consider the animatable layer re-ordering.
But the beauty of AS is in cutting down on all that drawing, so maybe practice more with economical character constructions drawn in AS - using simpler, more amorphous shapes that can be adjusted via point motion, to simply slide from face-on to the side etc.
Don't get distracted with switches, poses, actions, scripts; just get a handle on relatively simple, pliable shapes. To take a crude example, for a nose, hide the top edge of a 'o' so you have a facing-camera 'u' shape. When character turns to the side, move the points so it's more of a 'c' shape. The hidden edges are important here, because the solid fill of the shape is its 'form', which acts to obscure the 'far' eye behind the nose. Apply that principle to the whole character, and you've got a 90 degree turnable character.
Anyway to actually try and answer your question, the turnaround focus really suggests a 3D app. While not an exact match for your envisaged software, something like the old "Poser Artist" software might help. This was a cut down version of Poser that acted like an artist's mannequin. It won't do your drawing, but will help a lot with showing what needs drawing.
If you wanted a really simple modeller then check out the old but lush sPatch by a certain Mike Clifton. This AS-like spline modeller can be used to create your basic characters in a Preston Blair-esque forms fashion, letting you simply spin them around to see how the figures change in a turn. It's not meant for poseable models, and I wouldn't get mired in 3D export issues, but it is nifty as a model sheet style reference:
http://www.eatonhand.com/images/spatch.htm
More suited to your scenario, a 3D app designed for working with your own images is the open source Animata. This gives 2D characters more 3D-like rigging. Perhaps not as immediate as AS rigging, but if you're working with external images then you'll probably get a kick out of it. Check the nice write-up at CDM:
http://createdigitalmotion.com/2008/12/ ... c-windows/
Cheers, Nick.