The Checkin gadget displays the number of files currently checked out to the user in each site and gives the user the option to check them all back in. I have been using this in production for a few weeks now and it works well.
By default it shows just the sites that the user has files checked out. Clicking on the eye icon button shows all the sites.
You can download the source code from Github or use this url to add it directly to your Omni instance: https://jessgusclark.github.io/gadget-checkin/build/