An initial patch for review has been published by David Airlie that implements OpenGL Transform Feedback support for the LLVMpipe driver.
Gallium3D software-based driver hasn't reached OpenGL 3.0 compliance yet but it's slowly getting there. LLVMpipe is now in the process of picking up EXT_transform_feedback support.
OpenGL Transform Feedback adds in the capability of altering the rendering pipeline to allow for primitives processed by Vertex/Geometry Shaders to be written to buffer objects. By writing it to buffer objects, the post-transform render state of an object can be saved so that it can be re-submitted multiple times.
Transform Feedback was added to the core OpenGL specification in OpenGL 3.0 and it remains as of the latest OpenGL 4.3 specification with EXT_transform_feedback. Many of the other open-source Mesa/Gallium3D drivers have had GL3 transform feedback support for several months
David mentioned when publishing the initial patch
that the LLVMpipe EXT_transform_feedback support is currently passing 115 of 130 Piglit tests.