A feature that lets you choose which sources.list should be used

Use Case:
I have a network configured with approx, updates and programs are only downloaded once from the official repos avoiding unnecessary bandwidth use (really useful in third world countries where hi-speed connections are expensive). I also have a few laptops that when they are outside the office network can't install or update software, so a sources.list switcher would come handy in this case since the user outside the office network could switch the sources.list easily and update or install software outside the office.