The way this used to work before gpart, is that boot0cfg would send
a GEOM ctl-message to the MBR-geom asking "Would you please write this
boot code ?"
Since the MBR-geom was the "owner" of the whole disk, and the one
who had it open for writing, it could obviously do so, if it saw fit,
and after it had edited its idea about the mbr-partition table into
Gpart appearantly does not implement such a ctl message.