Is the website content somewhere in git? If so, I'd be happy to submit a pull request. Otherwise:
I had to say:
$this->bbcode_second_pass_code('', 'LANG="en_US.UTF-8" bsdtar -xpvf ArchLinuxARM-mirabox-latest.tar.gz -C root')
to avoid a bsdtar char set warning. I do have en_US.UTF-8 active on my host.
Also, the boot directory contains a dtbs directory, so the copy command after the untar should be
$this->bbcode_second_pass_code('', 'cp -r root/boot/* boot/')