I peeked into installer code. The bootloader seems to get installed here:
https://github.com/freenas/freenas/...f87/src/freenas-installer/etc/install.sh#L393
https://github.com/freenas/freenas/...f87/src/freenas-installer/etc/install.sh#L483
That would be the first place I'd try modifying. E.g. it uses partition type freebsd-boot, which would be interesting to change to something more traditional. Could it be old BIOSes don't recognise that? This is hugely guessing, but I'd start from there, tweaking options. That's easy place to try, as those are shell scripts. So you don't need to build anything, just modify the installer USB stick.
At least I have troubles seeing USB stick in BIOS. I don't remember if it was UEFI or legacy mode, the stick didn't appear in selection list for bootable devices. Could be the partition types is causing that issue.
Gpart manual is here: https://www.unix.com/man-page/FreeBSD/8/GPART/
https://github.com/freenas/freenas/...f87/src/freenas-installer/etc/install.sh#L393
https://github.com/freenas/freenas/...f87/src/freenas-installer/etc/install.sh#L483
That would be the first place I'd try modifying. E.g. it uses partition type freebsd-boot, which would be interesting to change to something more traditional. Could it be old BIOSes don't recognise that? This is hugely guessing, but I'd start from there, tweaking options. That's easy place to try, as those are shell scripts. So you don't need to build anything, just modify the installer USB stick.
At least I have troubles seeing USB stick in BIOS. I don't remember if it was UEFI or legacy mode, the stick didn't appear in selection list for bootable devices. Could be the partition types is causing that issue.
Gpart manual is here: https://www.unix.com/man-page/FreeBSD/8/GPART/