There's a reason I didn't elaborate on this option. It's complicated and not something I actually recommend. Also, I did the original exercise as proof of concept only, without attempting to formulate a comprehensive straight line installation method.
As a courtesy, I tried this evening to reproduce what I did a couple weeks ago. In the process, I bumped up against a problem I've always worked around, which is that one can't install third party software with secure boot enabled. Turns out, using the procedure specified in the installer mucked up my secure boot settings - maybe a bug in the installer, maybe a bug in my UEFI implementation - such that I'm now unable to boot in UEFI the USB drive I created two weeks ago (worked then), nor a freshly-created live ISO. Curiously, I have the problem even with secure boot disabled; only my old hybrid drives will boot in UEFI. Pretty annoying, actually, and a good demonstration of why disabling secure boot is widely recommended.
ETA: Following up, I was able to fix secure boot by reflashing the firmware. With that, the live ISO boots again and so do both secure-boot USB drives I've created (yesterday's and the one from two weeks ago). AFAICT, the problem is that the installer attempted to use MokManager to temporarily disable secure boot, but that went sideways for reasons unknown. On boot, I got an error, "Failed to open \EFI\BOOT\mmx64.efi ... Failed to load image ... Failed to start MokManager ... Something has gone seriously wrong [you think?] ... import_mok_state() failed." Did some research but wasn't making any headway, so went with the quick-and-dirty reflash solution instead. In retrospect, the easiest way to avoid this problem probably would be to disable secure boot in the firmware while doing the installation, then re-enable when done.
Having come this far, I will elaborate on two points. First, as mentioned, if installing in UEFI, the target will be missing a subdirectory for installation of the BIOS bootloader. Originally, I solved that problem by copying the subdirectory from a hybrid boot USB already at hand. This evening, I notice the subdirectory also can be copied from the live ISO. Open File Manager; right-click somewhere blank (i.e., not a file) and select Open as Administrator; from Menu, select View > Extra Pane. In each pane, navigate to /usr/lib/grub, live ISO on the left and target USB on the right; right-click i386-pc in the left pane and select copy to other pane. Close both sessions of File Manager (as administator and as regular user).
Second, for actual installation of the BIOS bootloader, I used something called chroot
. I'm not going to try to explain that here. For the time being, just copy these commands one line at a time (use copy and paste). In Terminal in the live session:
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$ sudo mount /dev/sdc3 /mnt # mount root
$ for i in /dev /dev/pts /proc /sys; do sudo mount -B $i /mnt$i; done
$ sudo chroot /mnt
# grub-install /dev/sdc --target=i386-pc
$ for i in /sys /proc /dev/pts /dev; do sudo umount /mnt$i; done
$ sudo umount /mnt
For context, I assume sdc1 is the BIOS boot partition, sdc2 is the EFI partition and sdc3 is the root partition. Ironically, my UEFI secure boot mishap notwithstanding, the BIOS bootloader installed without a hitch, which I was able to confirm on my BIOS laptop.
Don't know whether that's enough to get you over the hump, but it's what I've got. Frankly, the simpler solution would be to let go of secure boot.