option 1
Manage addon maps through a steam workshop collection. Have a helper script sync any updates / deletions from that one workshop collection.
This script syncs new, deleted, and out-of-date workshop items from a collection (I use to use it heavily when I hosted addon map servers):
https://github.com/dustinandband/ste...hop_downloader
Demonstration (rewritten in PHP - was a test just to show the script was in a working state).
option 2
Another option is to use rsync and sync addons from a central L4D2 server or github repo. Just keep the repo up to date and have the L4D2 server automatically sync updates from that repo periodically.
This is what my group does nowadays since there's only a few addon maps we use and we already sync server updates from a github repo every day.