I eventually found out what was going on. The FreeBSD boot problem was not related at all.
Long story short and for future reference, installboot needs to be run on the softraid volume, NOT on the physical disk. And this has to be repeated after a softraid volume rebuild in order for the new disk to be bootable too. This cannot be done from the boot media, but one can boot from media and then mount the softraid with the working disk and then chroot into that and run 'installboot sd2' (or whatever device name the softraid volume has). This was not obvious to me. Perhaps because with GRUB one has to install the bootloader and boot code on each single disk in a mdadm volume and not on the volume itself.
