Can't get 8.2-STABLE to boot from ZFS v28 with ashift=12

[Geoffrey Mainland]

I'm trying to get a system up and running with a couple new WD EARS
drives on 8.2-STABLE (built July 20, so it has ZFS v28 support). I'd
like to be able to boot from a ZFS pool where I've used the gnop trick
when creating the pool to make sure ZFS accesses my drive in 4k chunks.
Booting from ZFS without using gnop (so ashift=9) works beautifully, but
when I use the gnop trick to create the pool (ashfit=12), ZFS booting
doesn't work at all. I have a script to build my pool with and without
using gnop. Without gnop, I create the pool like this:

zpool create $TANK gpt/${DISK0} gpt/${DISK1}

With gnop, the pool is created like this:

gnop create -S 4096 gpt/${DISK0}
gnop create -S 4096 gpt/${DISK1}
zpool create $TANK gpt/${DISK0}.nop gpt/${DISK1}.nop

zpool export $TANK

gnop destroy gpt/${DISK0}.nop
gnop destroy gpt/${DISK1}.nop

zpool import $TANK

Other than this, the pools are set up identically by the script.

Without gnop I'm golden. With gnop, on boot I get the spinner and then
the computer reboots (it looks like boot0 fails).

Any ideas what might be going on?

Thanks,
Geoff