I'm just realizing that what I was trying to do was maybe nonsense : I was trying to pass on to the mysql server a value for the client on which my Qt program is running i.e. I was trying to "deceive" the server. And I could understand then that there would be no such method in QSqlDatabase (and that you couldn't understand my question initially).
Got it. The documentation quite frankly is bad written when it comes to the QtCreatorDeployment.txt What you actually write in this file is just the remote path and without the sftp://IP_ADDRESS so now I simply have:
and the binary from my Debian 8 lands right in there on the RPi2. I posted a detailed explanation on stackoverflow (link in my first post).