Currently no. Easy to implement though. What do you want this for?
When I originally posted this idea, this is the image I had in mind:
Two grass tiles, and two dirt tiles. Each one has a simple 50/50 chance of being dispersed. Anything more than that, and the tiling effect would probably drain a lot of FPS (which isn't worth it for such a subtle effect). And as long as you pay careful attention to how you design your tiles, it's possible to "break the grid" with only two tiles.
It's also the same thing I did for my grass
(before DF2014 turned my grass back into ugly ones ). Those things that look like 3 green dots are variants to break up the pattern a bit. Good to see you're also explicitly defining grass ^^
How extensive is the tile overriding functionality?
Can you override graphics by material? Quality?
Is there a test map to see all the overrides?
You should download TWBT and read the readme. It lists most of the tiles that can be overriden. It doesn't include tiles that are overridden in the raws (plants, monsters, units), so it's currently limited to items (food, clothing, weapons, crafts, etc), tiletypes (terrain features, etc. including trees, water, etc.), and buildings (workshops, walls, etc.). Can't also override by material or quality, but to a limited extent you can override by subtype (I posted the list a few posts down).
To see it in action, I recommend downloading the starter packs/lazy newb packs and examine the existing sets there (the override codes are found in the init folder, while the graphics are in the graphics folder). Spacefox is currently the most overriden graphics set, I think.