Thanks a lot [USER=30]@EdZ[/USER] for the simple and clear explanation 
Anyway, I solved the issue by sourcing, thanks to [USER=206]@Kmpkt[/USER], what appears to be a reliable flexible powered riser !

http://vary.technology/photo/RP22-WebShop.html
I have been in contact with their team and they accepted to sell a powered version of their riser on their webshop 
Very nice people to deal with.
I ordered a couple of those and I hope to get them to use soon enough.
[USER=15]@iFreilicht[/USER] I contacted Gigabyte support, and their very short answer strongly suggests the internal ATX_12V connector is acting as a direct passthrough :
To me, it means there is no voltage regulation prior to the ATX_12V connector (which name is confusing), such as the more "standard" Asrock H110 TM-ITX.
Which is a great news 