Build from local git repository

I would like to have drush build directly from a local git repository rather than downloading. This enables multiple developers to work with the same local copy rather than everyone downloading their own every time.

I have a single linux user, 'drupal' (configurable) which owns copies of the repositories. The script always
runs 'sudo -u drupal' to clone and fetch.

I also maintain a single local server, which runs 'git clone' and 'git fetch -t' against drupal.org and have
all other systems use the local one as a source (running git over ssh rather than http).

I have done this in perl already but I'd like to get it working in drush. Initially, I just need to be able to have drush unpack the code from a local repository. The clone() and fetch() features could be added later.

In perl this looks like (using 'zen' as an example):

$sitedir =  join( '/', $ENV{HOME}, 'mysite' );
$source = '~drupal/bare';
$name = 'zen';
$type = 'theme';
$tag = '7.x-3.1';
$prefix = join('/',$sitedir, qw( sites all ), $type);
$repo = join('/',$source ,$name . '.git');
system("git  --git-dir=$repo  archive  --prefix=$prefix/  $tag  |  tar xf -");

To unpack drupal itself, the $prefix is just $sitedir and everything works the same way.

I have looked at the drush code under commands/make/ but it will take me some time to figure out how to do this by myself so I would welcome some hints.