Not possible to use the 12V rail, maybe current is too high?
I know this response is old, but it seemed not yet addressed.
Short answer: no.
Long answer, for the EVGA power supply in standby mode, there is no 12v turned on. You can't make 12v act like 12v standby when there is no 12v present in standby mode.
As Shrek pointed out, since there is only 5v standby present with that EVGA power supply in standby mode, with a boost converter, 5 volt standby can be converted to 12v standby, assuming the boost converter works properly and both have enough current rating.
Still, I think the best cost-effective solution is to use the workstation's power supply with a 6-pin to 8-pin adapter. The wire you showed could easily handle over 500 watts. The rating of the pins is what will limit you to 200 watts; which is still way more than you need with the motherboard already providing 60 to 75 watts through the PCI-E slot.