In addition to playing around with a GUI front end for SSA I’ve been adding support for adding multiple audio files to allow the creation of chaptered audio books. Basically you add a new key in each chapter (audiofile=somefile.m4b) and SSA will add it to the output file; you add the chapters (images, hrefs or whatever) and SSA will figure out the length of each audio file and set the chapter locations automatically.
Now, there are a couple limitations: chapter’s can only start on audio file boundaries, all of the audio files must have the same format and you can lose up to one second of audio at the end of each file (because SSA does not deal in fractions of a second – this applies to audio imported for Enhanced Podcasts as well). This feature – sparsely documented – will be in the 1.0.1b version of SSA which should be uploaded this weekend.