It is possible. But I don't know how they plan on that working in practice. When I search through Kayak or the like and get re-directed to Delta.com I don't have the choice to select fare type, it simply takes me to the beginning of the booking screens.
Now if Delta started putting a buy-up option at the bottom then maybe. Something like "For just $xx more choose your own seat at time of booking, enjoy your Medallion benefits, etc."
They do. When you go to check out, they say that exact thing.
