I'm starting to think that it's going to be extremely difficult to get the desired effect by just animating the individual leaves of grass. It will not give me the ocean-like effect I'm after. I think I will need to work on a bigger scale, animating the 'waves' instead of every drop of water, as it were.
Failed attempt:
@Manupix: Thank you, I had not seen that ref.
@Decroded: Thank you, you're quite right about the rhythm. I need to avoid a constant pulse, I think. I think you could use layers to do moving shadows from clouds very well, without making it NPA. It's just a matter of combining layers, opacity and a limited number of colors. I don't have the skills to do it, but I think it would be possible.
@HDMD: Thanks! I'm surprised that no one has done anything like this yet. At least, I'm assuming no one else has done it, otherwise someone would give me a link
@lachrymose: That's an interesting idea, although I think I'll try to do the next version without animating individual grass leaves. If that doesn't work, I'll look at your suggestion.