List of all installed packages in a defunct Homebrew installation?
According to the Homebrew terminology, we have the following definitions :

Cellar :  All Kegs are installed here
Keg :      The installation prefix of a Formula
Formula:   The package definition
Homebrew installs to the Cellar it then symlinks some of the installation into /usr/local so that other programs can see what's going on.

As said in the comments, to list all packages previously installed on a backup drive you can do :
ls -1 /Volumes/*/usr/local/Cellar

If you want to have a better understanding of Homebrew paths, I recommend the reading of the following article : The Path to Homebrew.