Seems slowly the Broadcom chips start working:
I have used nobb1x 5787M kext, used the instructions provided to patch in my HW/MAC-address, modified the PCIid in Info.plist to match mine (i.e. pci14e4,167a). Using that I got a non crashing kext, that made the network card appear in System Preferences -> Network, with even notifying a network link properly.
However the interface wasn't "up", ifconfig did NOT report it as "RUNNING" as well. So I took the idea from arctelix above and searched Contents/MacOS/AppleBCM5787MEthernet for sequences with PCI-ids. I ended up replacing all 66 81 fa 77 16 byte sequences with 66 81 fa 7a 16 and ... except for the promiscuous issue I have a working network card now.
Thanks to all so far, maybe this final issue can be solved too.
Just noticing I must have picked up a different version of the kext from the Broadcom BCM5787M thread
Dell Optiplex 740 (MCP51 chipset, AMD X2 4850E)
Broadcom 5754 NetXtreme BCM5754