I thought I should try Papervision3D, an open-source ActionScript 3D engine, to see if I could use it for anaglyphic animations and games and such. So I it, and did their "Rotate" example, then turned it into an anaglyph:

I got it to work, but not how I hoped. I was hoping to have two cameras in the scene, a left and a right, looking at the same objects, and apply some kind of color filter to each camera, red for one, cyan for the other. Then, render both the cameras.

But it didn't work that way. I could put two cameras in the same scene, but I couldn't render them both. If I tried, only the second one actually rendered.

And so, to make this anaglyph, I actually had to have duplicates of everything. Not just two cameras, but two containers, two scenes, and two rotating squares too (since I couldn't register the same square with both scenes). It doesn't seem efficient, and it's almost exactly the opposite of my original goal, which was to create 3D effects one symbol at a time. So, Papervision3D might not be for me.

On the other hand, some other developers have managed to do very nice anaglyphs with Papervision3D. For example this works very well [1]. And this is quite good (be sure to turn "STEREO 3D: ON," in the lower right corner) [2]. And so is this [3].It is possible that they have hit upon something that I haven't found yet. So I won't totally write PV3D off yet, until I know more.

FOOTNOTES:
[1]: Ben Stucki (Ben Stucki did post his source code, but I think it's for Flex instead of Flash.)
[2]: Barcinski and Jeanjean
[3]: Mr. Doob