A few people are starting to post information on how to create custom Divi modules, including:
Jonathan Bossenger offers some good advice for building custom modules. He explains how to create a copy of an existing module and register it either as a new module, or as an overwrite of an existing module. He suggests starting with the existing module which is closest to what you want to achieve, and digging into the copied code to modify it to your liking (though no specific advice is given on how to do this).
