Based on the few things I've found others saying I was able to get Starling on top of Away3D, with mobile support. It is modifying the cores so hopefully it's integrated at some point.
1. Create Away3D View3d first and addChild.
2. Make sure the View3D has a background. Not sure why this is neccesary.
var bkgBitmapTexture:BitmapTexture = new BitmapTexture(bkgBitmapData);
view.background = bkgBitmapTexture;
3. Create Starling with the same context as Away3D.
mStarling = new Starling(Game, stage, viewPortRectangle, stage.stage3Ds);
4. Modify starling/core/Starling.as and comment out "RenderSupport.clear(mStage.color, 1.0);" in render().
5. Modify away3d/containers/View3D.as and comment out "_stage3DProxy._context3D.present();" in render().
6. Modify away3d/core/render/RendererBase.as and comment out "if (_swapBackBuffer && !target) _context.present();" in executeRender();
I'm not sure where to find the "last frame shader storing" referred to by sHTiF. Does anybody know where that is?