There is no other way except custom event handler. You will have to create event handler for same

OR

You can do a bulk checkin using the content and structure tool. From Site Actions menu use "Manage content and structure" or from the site settings page click on "Content and Structure". Navigate to the document  library from the treeview on the left pane. Use the toggle "check all" in the list view with all your documents.You should see a check  in option in the "Actions" menu in the tool bar.

Here is a bulk actions infrastructure for you to extend based on your needs. I developed this couple of months ago and applied refactoring to code as well so that anyone can write their own custom bulk actions.

zieglers.wordpress.com/.../generix-for-sharepoint-refactoring-applied-%e2%80%93-part-2/

Above article should have sample code and wsp for existing bulk actions - I believe 'Delete' and 'Checkout'. You can implement checkin functionality by doing single line code change. This solution is also using jQuery to enable multiple list item selection