http://spawnhost.files.wordpress.com/2011/05/multigrass1.jpg
http://spawnhost.files.wordpress.com/2011/05/multigrass2.jpg
You can have a lot of grass, in principle. The placement in these screenies is bad, it's just copypasted for demonstration without regard for natural distribution or variation.
Also, the type of grass is wrong for a large grassy plain - you'd use a different type of grass patch for that.
Pretty much the same rules for vegetation placement apply as in Crysis (which has many different kinds of grass, other objects like rocks, and many options for placing and rendering vegetation).
So, Quake can do large terrain (and also detailed, if you use 64 or even 32 unit blocks in CTE), and vegetation. You don't even need Darkplaces for that (in fact, a fast renderer like in RMQ or DirectQ is more important).
What it can't do is proper texturing of the terrain - textures don't blend in Quake unlike in Sandbox. Megatexture could help.