>>     tar cpf - .[a-z]* [A-Za-ln-z]* | (cd /mnt; tar xvpf - )
>> # the funny set of patterns just avoids dragging /mnt into /mnt/mnt

Just chiming in...

I used to do stuff like that and I still would for reasons other than the
purpose here.  I'd use it when splitting /usr/local onto it's own partition.

My alternate is to use dump/restore.  Similar incantation:
    cd /mnt ; dump 0uf - /dev/sd0a | restore xvf -

(or something really similar to that anyway)

The fun part about doing things this way is that it won't traverse the
filesystem and all the regex stuff is moot.  Then jump into /mnt/usr/mdec
and do the installboot magic.

Now back to your regular programming...