On A346 rows 26-29 (apart from AC+HK) can be obtained during online check-in by doing check-in on OS webpage. These seats will be marked in yellow color as "preferred." However, OS seems to be offering online check-in for purely LH flights on a "hit or miss" basis: no problem for me for FRA-KIX-FRA flights but could not go for FRA-SIN-FRA.
As NewbieRunner mentioned, in terms of comfort, all seats (apart from some in row 26) have the same legroom. The only benefit of the first few rows is that since the service starts from the front of the cabin, you are bound to get your meal choice.