You need to go to the "All Data Sets" object tree entry and create there a GOA script which will filter the data set names to only track the CATINDEX last qualifier. By doing this you will only get catalogs index information and then be able to react on over extend utilization.
The filter should be something like:
DSN INCL !CATINDEX! AND EXTENTS >= nn
Notice that you can do additional checking and get email alerts as other actions.
An IMPORTANT point is to take care to execute this script after the DTOC run ends to get the latest up to date information. This script is based on the DTOC collection data and could not run on Real Time.