A little over a month ago I blogged about El Capitan support for macosxbootloader and today, after 80 hours of research and development, I released two versions of boot.efi for El Capitan. One with a black background and a white Apple logo, and another one with a grey background and Apple logo.
You can read more about this project for El Capitan here.
Note: I should have added that using -f (flush caches) is no longer supported, and using it will throw a KP. In short. Please wait for me to add support for it (see development phase 3).
Edit: Great. Mike started a new developer thread over at macrumors.com
Have fun now!