I can confirm that it is possible to select an F seat when flying with a J ticket on aircraft that have it available and are not selling F as a separate cabin.

I phoned up CX as I couldn't select a seat on the first leg but thought I'd ask. I could have any F seat in the centre - not those on the window sides.
1A/1K are priority for those with movement difficulty. 2A/2K reserved for baby bassinet (as standard). Perhaps the window seats will open closer to travel but for now I'm happy to have an F seat.

Edit: this was free of charge, in response to a previous poster. All it cost was 40 mins on hold . The booking was an award through AAdvantage.

Will confirm success or not once I travel in November but for now, here's the booking proof. I have AY Platinum, but I don't think this had impact on whether I got the seat or not.