Hey @T1001,
I am still not 100% sure how you all mean that. But in short what you are doing is not possible in the public API and also only to some degree in the internal API. First of all, Cinema 4D only supports HDR in the viewport and it must be enabled by the user:
image.png
I.e., viewport effectively means Drawport API. What the docs write there about the only the viewport is only semi true, because you can embed a drawport into a GeDialog (but that functionality is still not even semi-public I think, but we could change that in a future release).
I am not quite sure what you mean by 'integrating a custom OpenGL pipeline into a plugin'. When I take this at face value, I would say you cannot do this. The Drawport API is an abstraction around effectively Vulkan and Metal. We do not write OS specific graphics API code anymore. This goes as far as that we have (sort of) our own shading language (MSL). You cannot just draw with OpenGL into a GeDialog, because that dialog will not be drawn with OpenGL and we also do not offer a direct access point to drawing handlers.
I assume this all has something to do with implementing some kind of render buffer? I really would recommend then applying for the Maxon Registered Developer program to get access to the Drawport API. Just as a warning ahead, the Drawport API is not totally undocumented but not documented to the same degree as our fully public API. It is an API from graphics API experts for graphics API experts. You seem fairly knowledgeable there, so this might be just your thing, but you have been warned, there will be less hand-holding there.
Cheers,
Ferdinand