I came across this thread as because I was looking to install Linux Mint Debian Edition on a flash drive and installing on compressed btrfs from the get-go. Although these solutions were not directly applicable to reaching my results I used some of this information to reach my target.
The problem was that the point of the installer formatting and mounting the partition and starting copying files was right next to each other, as so I was unable to perform the "remount" option mentioned by others above.
The LMDE version of the installer script was in python (usr/lib/live-installer/installer.py). I'm not sure if it is the same with Ubuntu, but if it is, this will be directly applicable. This allowed me to edit the script and add this line right under the line that origionally mounted the partition for "/"
os.system("mount -o remount,compress /dev/sda3 /target -t btrfs")
of coures the "/dev/sda3" will vary depending on your device.
I understand this is an Ubuntu forum, but like I said it came up with the search and this solution would be directly relevant if the installer is python based. We are all Debian here anyways, right!?