Allrighty.
So it's an "Economy Classic" ticket which does not come with free seat selection so I put it on hold for free for 24hr while I called QR.
Despite bewing OW Emerald (ie AS 100k) agent told me that he cannot assign any seat because this is an Economy Classic and it is currently on Hold. He tells me the ticket needs to be issued first.

End story:
There is no way to be 100% sure you can select your seat on an Qatar Airways Economy Classic ticket despite being OneWorld Emerald.
If I ticket it and call again after ticketing and they refuse, then there is no 24hr free cancelation (given the free 24hr free hold) and I am stuck with a ticket without preselected seat with no way of cancellation. I can't see verbiage on the QR website that explicitely says that you can select your seat while OW Emerald. I believe they say you are eligible for preferred seating, probably at check-in, but preferred seating in a middle seat for 15hr does not appeal to me.

Guess I am going to pass and use Turkish Airlines!
Minos