In the discussions so far, the idea put forth is to have pbuffers as user configurable render targets just like a display window. I don't know if any of the code to do this exists anywhere yet, but it was somewhat agreed upon as a good path to take.
i had pbuffers in 3dp at some point but i cut them out. (0.11 i think) i don't remember why exactly. possibly it had to do something with threads & context switches i couldn't get to work right.
i'm planning to dig into this again soon..
you can do a lot with multipass rendering. i use this in 3dp to fake a texture target: just render to a part of the buffer as large as the tex, and grab it. it's fast & convenient. the only drawback is that it doesn't work when the window is obscured by another one, but i think there should be a way to prevent this..