zfs boot vs loader

I am using ZFS for boot-and-root filesystem. gptzfsboot is used for bootstrapping,
if that matters.
Maybe the following is some kind of a pilot error, but I think that I see a
problem with selecting a different kernel using loader prompt.
That is, I escape to a loader prompt from boot menu. There I do 'unload'. Then I
'load' a different kernel, opensolaris.ko and zfs.ko. Then I 'boot'. Then root
mounting inevitably fails because ZFS root filesystem can not be found.

This is an unexpected result for me.
I wonder if something important for ZFS get unloaded when I unload preloaded
kernel and modules. Perhaps I have to manually re-load zpool.cache?

This happens with recent CURRENT, amd64.
What's strange is that I think that I can switch kernels from loader prompt
without any problems when using RELENG_7 and UFS-boot-plus-ZFS-root approach.

-- 
Andriy Gapon