YES!!! I decided to look at this problem one last time before moving on and then I struck gold; I finally found a download for the hard to get "WinFWUpg.exe" (which is a "Broadcom Network Controller Firmware Upgrade Utility for Windows NT 4.0, Windows 2000, XP 32/64, 2003 Server 32/64 bit Version 6.0.43").
With the commandline "WinFWUpg -w 1" I could set WoL to "on" directly in the NICs firmware. That worked and I can now deploy this change through SCCM to all Dell GX280s. I will call Dell again and hopefully they will update their solution database...
You can find WinFWUpg here:
http://www.ec-co.hu/drivers/teszt/Notebook/LAN/LAN10/FwUpg/Win/IA32/ Thanks especially to Sparkmaker for your insights.