XFCE uses the Thunar file manager, which can also be easily configured to allow for custom actions. Just go to the "Configure custom actions..." item in the "Edit" menu, and create a custom action for get and drop with the following commands:
For OS X, it is possible to get context menus in Finder. Due to how OS X deals with sym links, one needs to operate on folders if using indirect mode. Direct mode operation has not been tested.
1. Open Automator and create a new Service.
2. Using the Drop down menus in the top create the sentence "Service receives selected folders in Finder.app" to have it work on folders. For direct mode operation it is probably reasonable to select "files or folders".
3. Add a "Run shell script" element and fill in line with the following script:
The purpose of the first line is there to get git-annex on to the path. The reason for the for loop is in case multiple files or folders are marked when running the context menu command.
Finally save the the workflow under the name for which it should be listed in the context menu.