the power of broadcast
[a revolution in mass-scale content distribution]
I agree that this would be cool... however, the existing structure, especially the URL processing code in kast, would make this difficult.
Also, right now, the underlying API only supports "repeat broadcasts" for an entire folder (that's how the repeat BC config is stored on disk, with the path of the folder [or single file] being broadcast).
The API does support BCing an arbitrary list of files together, but a repeat broadcast must be grouped into a folder.
The upside of this is that you can change the files in a repeat BC simply by adding/removing files from its folder (though you probably should avoid doing this while a round of the BC is active so that you don't delete files that it is currently using.... check the "status" screen).
So, checkboxes will probably be added someday... but not right now. :)