I did not. Thanks for bringing this up. I will try it in the future, as part of diagnosing the issue.Did you try the old SMBUS pins blocking trick with the dell card?
Rest of my diagnosis from previous days is dumped below:
The card eventually worked in an ASRock AB350, so I have to assume there was some SMBus/I2C device on several of my machines that was interfering with it.
Strangely enough, on the machines where it prevented POST, hotplugging the card after POST then rebooting the PC, would bring up the card, and the card would function normally. This is the environment in where I tried to flash firmware from other vendors (just HP, since I couldn't find Intel OEM nor working Lenovo firmware). This did not result in a change in behavior.
I was unable to disable flash or disable the various boot modes on the card, since bootutil (Intel) refused to execute those commands on this card. I noticed the Intel OEM card had flash disabled (which also disables the boot modes). I tried using a firmware dump from my working Intel OEM X520 card, but Intel's tools and ethtool did not allow me to flash the Intel OEM firmware dump to the Dell X520. The Intel OEM firmware was noticably smaller than the Dell or HP firmwares.
I could patch each octet/byte manually via ethtool -E, so I started working on a script to do that, before finding out the card worked normally in my AB350 board (the only AMD machine I tested it in - I don't know if that is relevant).