I happen to have such a card, which I'm unable to use in my Opnsense box (!@!!#).
I like the idea using it stand alone. So much so, that I have just purchased a PCIe Riser for Bitcoin mining .
(There was no PCIe backplane available on Amazon and this is also really cheap for 14EUR.)
If it arrives until this friday I will test it this weekend, and report back.
Why did I not think about this before?
Ps. the issue I see is the standard ATX power supply, which will be very inefficient for this low consumption setup and also the size of the PC case.