@Error014: As always a joy. :3
It's just an idea, but I think SSAA could heavily improve the image quality by smoothing out the aliasing introduces by transparent textures. SSAA is usually too slow for practical use, but since your scene doesn't use any computational intensive pixel calculations it shouldn't be a problem to render the scene at four times the resolution (2x2 SSAA). This wouldn't hurt the blocky style at all but gives you very consistent results.

Sorry but I've to show some of my graphics poser stuff too.^^°
Creating normalmaps in frequency domain is awesome, since you can compute the slope vectors analytically and you can weight how much which frequencies contribute to the result.

Base texture:


Normalmap computed in frequency domain containing fine grained and coarse information:


And it's also possible to recover heightmaps from normalmaps accurately:


I think I've to create a normalmap tool using this techniques. The results are really good and it's damned fast: you could adjust a curve which weights the amount each frequency shall contribute to the result while watching the changes in real-time (as I wrote earlier, the whole process of bringing the image in frequency domain (fft) and back (inverse fft) takes about 2.3 ms on a GTX 560 Ti (fps = 1000/ms))