If you are going a tiny bit of shader coding in Stage3D, you must feel the same pain as we do when you want to do anything remotely complex.
We searched a lot for some kind of usable solution in the past, the best we could found glsl2agal but that's not supported by adobe anymore and didn't feel very easy to integrate with Starling.
In our dreams, we wanted something that can be compatible with some kind of node shader editor like the very nice ShaderForge for Unity or the one from Unreal.
We really couldn't find anything and we are very lucky to have an very good fx engineer capable of working in pure agal and still achieve some impressive results. As an example, we have this effect https://github.com/xissburg/XBPageCurl running in Starling in 1 draw call and 60 fps on mobile (and still 60 fps when we have the full game below), with interactive & dynamic content on each page.
Anyway, I wanted to share with you a project that we recently found out called OGSL https://github.com/saharan/OGSL. It's a high level shading language for AGAL similar to GLSL, compiled at run-time and easy to integrate with the as3 side of your program.
You can create custom functions, do if and loop, pass named variables etc..., with the same limitations/workaround as AGAL of course, but it look like proper high level code !
Unfortunately the website is in japanese and information is quite limited but there is an online editor to let you experiment http://el-ement.com/blog/wp-content/uploads/stage3d-shader-editor.html and some source code that makes everything quite clear (at least to me) http://el-ement.com/blog/wp-content/uploads/stage3d-shader-rt.txt
I hope others will find this interesting and maybe daniel, you could take a look at integrating it with starling somehow? In my eye it could fit quite nicely with Starling 2.0 idea of custom renderer and custom vertex/fragment variables.
What do you all think about it ? For us it looks like a big deal.