i see. i'd wondered after i'd posted whether this might be it:

the kernel driver doesn't work out partitions on its own any more.
the devices' partition tables are set, for instance in /bin/termrc, by
disk/fdisk -p and disk/prep -p, into /dev/sd??/ctl, but that's not much help if you need the
file systems to get termrc (let alone fdisk or prep), and you need the partitions
to find the file system.  thus, boot adds the results of its own probe to the
internal copy of plan9.ini passed to the kernel, and so the partitions are
defined in time for boot to find them.

/sys/src/9/port/devsd.c has
		 * Use partitions passed from boot program,
		 *	sdC0part=dos 63 123123/plan9 123123 456456
		 * This happens before /boot sets hostname so the
		 * partitions will have the null-string for user.
		...

you could configure fdisk and prep into /boot, and have boot use them, but perhaps the
easiest change for you might be to stop 9load from checking the values (temporarily)
until something better is done.