Recently I have got to those code corners again and decided to play with it more. It's using fractal channels to generate the bumps, subjecting it to several treatments - the effect being smaller on some types of grass, and also changing together with modulating colors.
Here's the result:
For comparison, here's how the same scene looked until recently:
The effect is most visible when the sun is lower, it's achieved just by normal lighting. It should be probably combined with another effect that will make it more attractive during noons - shades from grass blades, visible from the side.
Of course, an important thing will be to combine it with real 3D blades smoothly appearing up close, and to apply the effect on other types of vegetation visible at distance.
A short video showing the thing in motion: