When I said “Foundation isn’t releasing the firmware to take CSI input “past” the camera tuning ISP and the information that you would need to do this yourself is Broadcom proprietary” I was definitely over simplifying.
What I really want to say is all firmware necessary to interface an HDMI to CSI converter chip with the Raspberry Pi that can only be written by someone who has access to proprietary Broadcom information. Obviously, someone must have written this for your HDMI input prototype to work. But Gordon says the Foundation will not release this firmware at this time or possibly ever because it would then need to be maintained by the Foundation. If this firmware were available, a third party could write the HDMI 2 CSI chip configuration firmware.
Early on in this thread, some discussed the possibility of creating hardware that would take an HDMI (or some other) input and mimic exactly the output of the Pi camera. As you pointed out, a lot of tuning went into the input ISP for the particular CCD in the Pi camera. So a lot of “reverse engineering tuning” would be necessary to exactly detune video exactly right so that the Pi ISP then tuned it exactly right again. That sounds a little psychotic to me. The HDMI 2 Pi board would need the equivalent of the input ISP, another video DSP chip or FPGA. Read early on in this thread. James goes into some detail of the amount of parameters and the “art and science” of tuning the ISP for a specific CCD.