In OpenBSD the PBR (partition boot record) is responsible for load /boot in memory. The code of PBR is called biosboot. The configuration of biosboot is handled by installboot program. I have use with the device of the system and it works fine, but I tried to make it with another device (a flash memory) and it didn't work. Maybe I didn't use it correctly, so I have created a script which makes that work. I have called ib.sh, and we can use as follow:
# sh ib.sh /dev/sd1c 3
where /dev/sd1c is the device and 3 is the inode number.