Comment 1 for bug 1317054

I don't believe there's a good way to do this right now. apt-setup is the installer component where this should probably be fixed.

In the meantime the simplest suggestion I have is to use preseed/late_command to append lines to /target/etc/apt/apt.conf. That won't cover the situation where you need to install some packages from the corporate repositories during initial system installation; if you need that then let me know and I may be able to put together a rather more complicated piece of preseeding to do the job.