You can select better seats on discount economy if
That's what I found out:
Even if the seatmap is greyed out, you can tap on seats using the website.
If one is available it will display its price.
BUT
If you wait till about 90 to about 40 minutes to departure, free seats, even so they would have cost extra, are all selectable at no extra cost.
I.e. Seat 12A bulkhead was available for about €90.
Shortly before departure it was still available at no charge.
And so were plenty of other seats.