I just finished using my MBP to download 10.11.3 install app and create a Legacy Mode USB installer for El Capitan, choosing my formatted USB, El Capitan, Legay Mode and Inject nVidia (for the 9800GT GPU) and building it.
I the inserted it into a USB port on the back of my GA-EP45-UD3P Rev 1.6 board with BIOS FE and booted it with F12 to select the USB. Erased one of my test drives and installed El Capitan with absolutely no problems, rebooted to the USB again, selected the HFS+ icon and booted into OS X setup. Finished setting up and creating a user and got to desktop.
Went to
http://www.tonymacx86.com/downloads.php?do=cat&id=3 and downloaded MultiBeast for ElCapitan and made these choices:
built and installed it. Ejected the USB and removed it, then rebooted. Opened Preferences and set sound to Internal Speakers Built In. I now have audio and networking and it is booting from the hard drive.
No DSDT required at this point, but I still need to check sleep/wake and power management.
EDIT: Short term (1/2 hour) auto sleep/wake working, going to leave it on overnight ans see if it wakes tomorrow.
Intel Power Gadget refuses to install - says unsupported CPU. Have to try something else.
EDIT 2: Sleep/wake over night works.