Happy to be of service!
By chance, yes I am! 
you know him then, I take it?
Ha! Very small world, I spoke to him last week. Perhaps it might be worth looking if there is a solution to the Blackmagic issue. We’re looking for a ROV drone next year for some shoots.
Having done a bit more testing, I can confirm that adding hypersmooth to the mix greatly increases delay from the GoPro hero 8 as well if the camera is recording video.
However: the GoPro hero 7 has a 2-2,5 second delay, even if the camera is not recording. The Hero 8 has a 2-3 second delay when hypersmooth is enabled AND the camera is recording.
If the Hero 8 is merely on, and hypersmooth is enabled, the output from the HDMI-port has low latency.
I have built something along these lines. I had some of the same requirements as you guys for a GoPro that can be viewed live on the surface while it is recording to monitor the status, and also to fully control the camera (on/off/start/stop/change modes/etc…) remotely via rs-232 or 485.
I know its old tech now, but I heavily modified the Hero4 black to reposition the image sensor to be inline with the enclosure, make all controls available remotely, live video out (CVBS), add heat-sinking to the camera, powered externally (7-32vdc), internal battery backup (UPS) in case there is power loss it will gracefully shut down, and a compact housing designed for well over 2000’ depth.
I made a number of these for different projects and plan to make about 15 more at the end of the year.
They can run a user defined script to operate however you want. For example it can power up/down with the ROV power and automatically start/stop recording for simple hands off operation. It can be manually controlled from the surface controller, or it can perform more complex routines, such as power up take a photo and then record a video for 5 minutes, then power down, and repeat that every hour for as long as the SD card lasts.
I have put seacon connectors and BR penetrators on them, and just built one with the Blue Trail connector, which works well.
The next version may also include a small plug to remove the SD card without opening the housing.
The overall dimensions are about 6.5" x 3" x 3".
These work great for science, and I also use them on my towed camera sleds, as well as some other very deep water applications.
If anyone is interested in more info let me know.
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Or you could buy one of these…
Live video out, low latency (ms), 4K recording on SD card.
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That’s a very nice setup. I’m building out a towed tube that is likely going to use an rpi and rpi cam at shallow depths and would love to see more of your design including how it attaches and dissassembles.
Please excuse my ignorance on this - I have only just ordered my first BR2 and have never looked at Raspberry Pi before. I see this thread has been going for many years now so ultimately wonder if new technological solutions have come about for mounting external cameras?
I see that Camera Tools For GoPro have a means of controlling multiple GoPros with a Pi by joining them through USB https://youtu.be/ESEuJ6KsK24?si=XZD9EMDB7VAHL2fu
If using the ethernet switch in the ROV and GoPros connected to the Pi by USB, could GoPros be triggered through the method shown in the video link? I am not interested in a live view from the GoPros, just want to be able to trigger them. Additionally I have a Canon R6 mk2 that I would like to do similar with.