About the only way I can think of to fully automate this is with a custom SolidWorks addin.
Other option would be to either setup a search or that would list all the files checked out by a user. The user could then run the search before they leave for vacation or have them do it at the end of the day. They could then check in the files directly from the search.
If you are not aware, you can define a search and save it as a favorite.
Thanks for the reply, Wayne. I suspected that was the answer, but my mgr wanted me to be sure.