Although… 
a bit more testing with the Rift and I find a potential issue.
Having a sharp contrast with the dithering does not look good. So, light green grass against light blue sky is great, but dark shadowed grass against light blue sky is nasty. Not only because you see the dithering which is much more apparent at the lower res of the Rift, but I think there’s something odd about the stereo effect. It looks kind of “fuzzy”. And now I remember reading somewhere that Temporal AA should be disabled as a post-process for the Rift as it is not supported and causes strange visual effects. I guess this uses the same mechanism. Potentially I can mask this with some more anti-aliasing and setting the lighting to avoid sudden changes of contrast where they can be seen.
Otherwise we’re back to plugging the gradient into OpacityMask without the dither, which has its own issues. But maybe I can also manipulate the colour of the grass as it fades so the lower part of the stalk is brightened slightly and matches my landscape texture. Or maybe set up my alpha to fade in horizontally instead of vertically for each blade.