Can one choose a specific repository for the Ubuntu setup process to use when installing from the live CD?

I know a very fast repository I often use to install packages, I would dearly like to use this when I am setting up Ubuntu, the mirror used by setup when the region is choosen is usually very slow, this results in apt configuration taking a long time.

I usually have to remove network connectivity from the machine I am setting up and the setup process will quickly complete. I then use synaptic to change to the fast repository before installing all those large packages I need, it would be much smoother though, if I did not have to do this and could feed the setup process repository information before hand.

I am wondering if I use the synaptic package manager in the live session and select a repository if the selected repository will be the one used during setup or will the setup process use the one it thinks is best usually a local mirror?