I decided to modify a PCI riser cable, the type of riser is:

http://www.aliexpress.com/item/Free-Shipping-PCI-Express-1X-to-16X-Adapter-PCI-E-Riser-Card-Flexible-Extension-USB3-0/1677878410.html?spm=2114.01010208.3.105.kPH12c&ws_ab_test=searchweb201556_0,searchweb201602_1_10037_10017_407_10033_406_10032_10040,searchweb201603_8&btsid=38dba5c6-133a-461e-9c9b-8bf58a67dd77

The power is only 12v input power from a server power supply, there is no 5v output from the server psu. The riser requires 5v input in order to regulate the voltage down to 3.3v used by the PCIe signal bus.

So this unholy creation came to life:

http://i.imgur.com/3NSOXXE.jpg

http://i.imgur.com/vr2MzyS.jpg

http://i.imgur.com/hoaWe3I.png

The GPU is now only powered by a 12v server psu, the step down buck converter takes the 12v. Much to my surprise, it actually works. Currently testing 370 and 380x with it. There might be issues with grounding, to be sure one should share the ground on the 12v output side for multiple power supplies.