After many restarts I know what to do to have a fully functioning install of El Capitan.
13. Open /Library/Extension on the new SSD and copying the new kexts
FakeSMC.kextAppleIntelE1000e.kext
AtherosE2200Ethernet.kext
RealtekRTL8111.kext
from there to /EFI/CLOVER/kexts/10.11
14. Not all this kexts are needed for the Z77-DS3H. Therefore all but FakeSMC.kext and realtekALC.kext are deleted.
15. Now the kext for ethernet "ALXEthernet.kext" is copied from the EFI partition of the USB stick (use EFI Mounter-v2.app) to the EFI partition of the new El Capitan SSD.
16. In /E/C/k/10.11 now are this three kexts:
ALXEthernet.kext
FakeSMC.kext
realtekALC.kext
iMessage works perfectly on this board. Note make sure to inject System ID due to SId Bug.Well, if you read you see that my installation works completely...
I only can tell this. Maybe you want to try my method on a second drive?
It's very simple to replicate.
iMessage etc. is a completely different theme. You have to search explicitly for this.
13. Open /Library/Extension on the new SSD and copying the new kexts
FakeSMC.kextAppleIntelE1000e.kext
AtherosE2200Ethernet.kext
RealtekRTL8111.kext
from there to /EFI/CLOVER/kexts/10.11
14. Not all this kexts are needed for the Z77-DS3H. Therefore all but FakeSMC.kext and realtekALC.kext are deleted.
15. Now the kext for ethernet "ALXEthernet.kext" is copied from the EFI partition of the USB stick (use EFI Mounter-v2.app) to the EFI partition of the new El Capitan SSD.
16. In /E/C/k/10.11 now are this three kexts:
ALXEthernet.kext
FakeSMC.kext
realtekALC.kext
Please do NOT use ALXEthernet.kext. It is BROKEN with this motherboard (or just broken, period). You will randomly get crashes while transmitting ethernet packets (receiving works OK, so if you never upload, you may never trip over this bug, but it's there). I put up with this for ages, and it was horrible, before eventually figuring out that my ethernet was the problem. Then I got a USB Ethernet which fixed things for me at the expense of performance. I finally found the right driver while I was upgrading to ElCap.
There were apparently two versions of this motherboard, one with the Atheros 8151 and one with the Atheros 8161.
I have the Atheros 8151, and I'm using the AtherosL1cEthernet.kext downloaded from http://www.insanelymac.com/forum/files/file/374-atherosl1cethernetkext but you can compile it yourself if you like from code at https://code.google.com/p/iats/downloads/list .
If you have the 8161, you'll need the AtherosE2200Ethernet.kext. It's safe to just put both in /L/E, or in /E/C/k/10.11, if you don't want to figure out which one you have.