The miniroot on solaris 10 in update 6 is a UFS image containing files compressed
with the utility "/usr/sbin/fiocompress", which is currently only available on sol10u6

mount the miniroot with somethine like:
    cp -p /path/to/sparc.miniroot /path/to/sparc.miniroot.new
    lofiadm -a /path/to/sparc.miniroot.new
    mount /dev/lofi/1 /tmp/a
unpack /tmp/a/.tmp_proto/root/etc/default/nfs
with the command:
    /usr/sbin/fiocompress -d /tmp/a/.tmp_proto/root/etc/default/nfs /tmp/nfs
then edit /tmp/nfs
and add after 
    #NFS_CLIENT_VERSMAX=4
the line:
    NFS_CLIENT_VERSMAX=3
recompress the file with
    /usr/sbin/fiocompress /tmp/nfs /tmp/a/.tmp_proto/root/etc/default/nfs
umount the miniroot with:
    umount /tmp/a
    lofiadm -d /path/to/sparc.miniroot.new