Yeah... I have since some time these notes for a blog post
virtualenv -p python2.7 /tmp/virtualenv
cd /tmp/virtualenv/
./bin/pip install virtualenv==1.7.2
mkdir /tmp/pypi-local
wget https://pypi.python.org/packages/2.4/s/setuptools/setuptools-0.6c11-py2.4.egg
wget https://pypi.python.org/packages/25/57/0d42cf5307d79913a082c5c4397d46f3793bc35e1138a694136d6e31be99/pip-1.1.tar.gz
/tmp/virtualenv/bin/virtualenv -p python2.4 . --extra-search-dir=/tmp/pypi-local --never-download --no-site-packages
./bin/pip install https://pypi.python.org/packages/84/65/1d3601dab2bc3c539b417e42ccfb249c843632044afd27cc18c1e96b526a/zc.buildout-1.4.3.tar.gz
The trick is that first you install an old virtualenv version for python 2.7, then you download locally old version for pip and setuptools, then again you bootstrap a python2.4 virtualenv using that files.
Of course you need to override the buildout index:
[buildout]
...
index = https://pypi.python.org/simple/
I hope it is clear enough 
If not I will write a blog post 