@KristofferLislegaard thanks for the information.
We got that resolution working with the Blackmagic Intensity Shuttle:
The Organelle HDMI output has worked on so many other devices (capture cards, monitors, tvs, projectors, HDMI-to-Composite converters). Some of the Blackmagic gear must just be too picky about the source video since it is the exception and not the rule.
Sorry to bring up this zombie thread, but this was helpful for me. Probably common knowledge, but here it is.
Opened the command line, remounted the boot folder so it wasn’t read-only by typing ‘sudo mount -o remount,rw /boot’ .
Edited the config.txt file in the /boot folder by typing ‘sudo nano /boot/config.txt’ .
I went down and uncommented the two framebuffer lines by removing the # at the beginning, though you might have to mess with the overscan or the resolution numbers. ‘Ctrl+x’ to exit, ‘S’ to save, ‘Enter’ to save it under the same name. Then just ‘sudo reboot’ to reboot it. As I understand it, it’ll revert back to read-only upon reboot.
That’s it, hope this helps future me when I have to do it again.