Traditionally "megatexture" style terrains simply mean that all texturing is done by creating a single unique texture for the entire world, rather than using weight-maps and repeating textures for example. World Machine 2 has always been capable of creating megatexture output via the tiled terrain support -- indeed, Quake Wars was one of the first titles to use mega-texturing, for which WM was used to help them create a base procedural texture for their world.
I believe what you're referring to are the Scatter and Placer devices, new to 3.0. With these, you first import your textures in localspace (checkbox in the file input device), then wire from that to the placer/scatter. Placer lets you repeat a texture across the world, while scatter randomly places instances. Using both, you can create a base repeating texture layer, then break it up by randomly inserting variants/translated/scaled versions across the world.
Place and Scatter (and localspace support in general) are the primary "theme" of the 3.0.dev1 release. You can both create your own localspace objects/textures, or import external ones into WM.