Im in the same boat. same update. Asrock Exterme6 X79. iMessage died too.HI I have old Asrock Z77 extreme 4 with Broadcom BCM57781.
Till now I use FakePCIID.kext+FakePCIID_BCM57XX_as_BCM57765.kext
and I was't any problem.
After I update OS X from 10.14.2 > 10.14.3 and Network gone.
If any one have same problem and did some one have solution ?
Its great to hear you got it to work!I fix it but I forgot what I do.. actually work with FakePCIID.kext+FakePCIID_BCM57XX_as_BCM57765.kext. but you should touch clover... tomorrow I will upload EFI