To allow to list specs in a test browser, a method should be exposed that returns all specs that will be run when calling testbox.run(), testbox.runRaw(), or testbox.runRemote().
The UnitRunner.cfc obviously already has a private method getTestMethods(), which returns all specs for a given bundle. So, the easiest solution would be to simply make this method public.
Then you would have to call getBundles(), instanciate each bundle and pass it to the method to get the specs.
An advanced solution may be to add a method (probably to the testbox component) that returns all specs at once grouped by bundle name.