> >From the boot_console manual pages, I gathered enough information to
> rebuild /sys/arch/i386/stand/biosboot, but how do I actually install the
> new bootblocks?
>
> I tried installboot, but that did not work.

how did your command line looked like?

should be

/usr/mdec/installboot -v <path/to/created/file.sym> /dev/rwd0a

replace rwd0a with something else if needed