ZFS pool bootfs property and vfs.root.mountfrom

Hello, zfs gurus!

I wonder, is it hard to implement some kind of auto-sensing 
vfs.root.mountfrom from zfs's pool's bootfs property, which normally 
points to dataset that should be root FS?
I dig into sys/boot and find out that we "just" :) need do place 
setenv("vfs.root.mountfrom",ZFS_GET_POOL_PROP("bootfs") <- some kind of 
metacode in a RIGHT place. But by all means I cannot gather whole 
picture of freebsd booting process (mean in sources who calls what) and 
find that magical RIGHT place. So may be zfs-gurus can say something? Is 
it possible to implement such thing? Is it hard? May be some hints of 
what to dig deeper in?

thanks!

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    AVR39-RIPE