Workaround

1. Unpack boot archive:
/boot/solaris/bin/root_archive unpackmedia /export/u8 /export/mr
2. Modify the miniroot zpool command to insert a cool (^^) hack:
mv /export/mr/usr/sbin/zpool /export/mr/usr/sbin/zpool.orig
Create the file /export/mr/usr/sbin/zpool, with this content, and make it executable:
if [ "$1" = "create" ]
then
echo
"########################################" | tee /dev/console
echo "# Hacking zpool version" | tee /dev/console
       echo "# Parameters: $*" | tee /dev/console
/usr/sbin/zpool.orig `echo "$*" | sed 's/create/create -o version=10/'`
echo "########################################" | tee /dev/console
else
        /usr/sbin/zpool.orig $*
Be warned that it is a quick and very dirty hack.
3/ Pack boot archive: /boot/solaris/bin/root_archive packmedia /export/u8 /export/mr

And voilà, now you can jumpstart with ZFS flasharchives.

PS: I also hacked the zfs command, in this way:
if [ "$1" = "receive" ]
then
        /usr/sbin/zfs.orig $*
echo "Hacking ZFS mountpoint for rpool" | tee /dev/console
rvol=`/usr/sbin/zfs.orig list|grep "ROOT/"|grep -v @|grep -v var | awk '{ print $1 }'`
       /usr/sbin/zfs.orig set mountpoint=/ $rvol
echo "Set mountpoint=/ for $rvol" | tee /dev/console
lse
        /usr/sbin/zfs.orig $*
It's useful if your rpool/ROOT has "legacy" for mountpoint instead of "/". In this case you get an error after the flasharchive extraction during the Jumpstart. You would get the error "cannot stop the extraction".
Warning this script is not generic, the way to determine the boot volume is VERY dirty, but works for my environments ^^