I would do this:
1 .Submit the disk analysis RTVDSKINF that comes with OS
2 This will create a physical file QUSRSYS/QAEZDISK. There will be a record in it for each object in each library on the system. Among the fields are the date the object was last used, YYMMDD, and a century field. I would query this file, concatenate these two fields and then create a summary record by library with the maximum of this value (option 4 in the query summary functions.). Output to a file, list that one in ascending order by this summary field, and your oldest last-used libraries are at the top of your report.
3. Enjoy your new system!
I have done this a couple of times in the past. Get a list of objects that have not been used in a while. Create a holding library and move the objects to this library. Observe your applications for a few days/weeks. If eveything is OK, then back up the library to tape and delete it.