Hi. You'll probably have to wait until CE gets back for a definitive answer here. As I am not envolved in that I can only assume that it would depend on library and the community of Panda3d.
Taking OGRE and Irrlicht for example, both have fairly large followings and are open-source. I looked at the Panda3d site and it looks like it has a decent following in addition to being open-source, so I think you can make a good case. Especially if you are going to write the initial renderer yourself
Does Panda3d also have it's own resource-system? If so, you may want to look at creating a custom ResourceProvider for Panda3d as well.
In regards to writing renderers I'm not aware of any particular essential document, but the renderers CEGUI already provide should be easy enough to port. I haven't looked at the v0.7.1 renderer code yet, but I can tell you the v0.6.2 OpenGL renderer was fairly simple.