You can do this with the Folder Search on the new 4.0 Beta. Ask to be added to the beta program and to get permissions to the Beta Forum.
Create a Folder Search clip, set its Playback Settings to Auto Rewind and Loop. Set its Template | Transition to whatever you want (e.g. Fade, Medium), and chooseTemplate | Links | Add | Timer: Next Clip, desired number of seconds in Link Time. Even when it's playing, you can drop new files in, and they'll be added to the end of the cycle. Removing files when playing works too, as does replacing a file with a new version with the same name.