You shouldn't see any problems using powering the GTX970 via both the PCIe slot and the HDPlex.
The best solution in my opinion would be what I did, supply all 12V via the HDPlex, but that would mean modding the 3M riser.
What a strange thing they have done with the 12V EPS connector and the input range.
It used to be 12V to 24V on the DC jack and the internal 2pin connector.
Now it seems to be 19V to 24V on the external DC jack and 12V only on the internal 4Pin?
Edit:
A quick browse through the manual reveals that it can still output the DC voltage, so it seems it can still be used as a passthrough, so it can be used to power the HDPlex.
However as an input it can only do 12V. Perhaps it circumvents or is after the 12V stepdown converter?
Here's the manual, look on page 13
http://download.gigabyte.ru/manual/mb_manual_100tn(gsm)_e.pdf
