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The EFI boot picker is Apples way to select the boot device on power on. There is no (visible) PC BIOS which one could access to rearrange the boot order if something fails on the standard boot drive. The complex thing with iMacs compared to MacPro5,1 or modular PC systems: You have to open an iMac, disassemble glas, LCD panel, numerous cables just to disconnect or attach internal drives. Trust me, if I think about opening one of my iMacs again a get a cold shower on may back, it is a real pain. There is always the risk of breaking some parts and the iMac (27") is a heavy beast.
OpenCore and the rise of the OpenCore Legacy Patcher would make it (in theory) superfluous to have an EFI boot picker. OpenCore provides it's own version which works reliable. Unfortunately it depends on a piece of software residing within the EFI partition of one of the internal or external disks. When replacing, updating, editing the config in the worst case one can end up in a situation where booting fails.
Unfortunately there is no other way to force a boot selection of all attached internal SATA and external USB devices than using an EFI boot picker. The only noticeable exception is pressing and holding the C key after power on and during the boot phase forcing the system to boot from the internal DVD (but this needs the DVD to be in good shape). You need to create an bootable OpenCore CD in advance. Another way out would be using a FireWire cable and using the Target Disk Mode (by pressing and holding the T key during power on and boot) and mount the internal disks of your iMac from another Mac and fix the software problem. Both solutions need additional hardware - which is not in place in any case.
After getting all information together I applied the process to create EFI BIOS versions for most of the GPU cards listed on the main page. My plan is to describe the complete process and filling the gaps of missing pieces with screen shots and more written text. I have no fixed script yet so the structure and page naming may change once in a while.
After finishing the programmers documentation I will publish all data and BIOS versions...this may take some days. No, I have no plans to create a video to visualize parts of the process, however I may refer to some videos if I find suitable ones.