I find that even in single-user mode, at security level 0, installboot complains
about "read-only filesystem" when writing the disklabel.  This is when
I invoke it as follows -
	/usr/mdec/installboot /boot /usr/mdec/bootxx /dev/rsd1a
(note that this is on a system which has root mounted from sd1a)
which seems to me ought to be the correct way to invoke it.  As I said, this
is with the machine in single-user mode and / mounted read/write.  The 
installboot binary being used is that from the 11/27 snap.
-ben