long time member here - always been repping some of the more hard to accomplish projects. I achieved a flawless Yosemite build with my old Z68 motherboard, using legacy Award Bios, and tomorrow I'm taking the journey to El Capitan when I get my spare Solid State.
If anyone else has this board still, or the Z68 in general, and wants to come along for the ride, be sure to stick your head in this thread and I'll do my best to answer any questions.
I've done the initial prep and managed to boot into a USB and run a test El Capitan install (Faulty spare hard disk though so couldn't boot into the OS after).
The only thing I had to change since my El Capitan build thus far, was disabling CPU EIST in the bios. It's not so straightfoward to find, but its in advanced CPU settings a few pages in.