Just for reference cause I had to figured it out again after upgrade from Mint Lisa to Mint Maya (which in Mint world means reinstalling whole system).
This is the way to create cowbuilder environment with Debian Sid/unstable under Linux Mint Maya or any Ubuntu based system:
sudo sed -ie 's/MIRRORSITE.*/MIRRORSITE=http://ftp.debian.org/debian/g' /etc/pbuilderrc
sudo apt-get install debian-archive-keyring
sudo cowbuilder --create --distribution sid --debootstrapopts --keyring=/usr/share/keyrings/debian-archive-keyring.gpg