Some people use etckeeper to maintain a log of changes to /etc.
On RH-like systems etckeeper fires a commit as part of a yum package operation (using a yum plugin).
It seems to me that as part of that you could dump the current list of packages - basically the output of 'yum list installed' - into a file within etc (so under etckeeper) which would give you what you were after (you already have the yum repo configuration in the etckeeper repo).