I was thinking that the Spellsword class would benefit from the addition of the Combat Casting feat upon taking the class.
Since they train to learn to be able to cast while wearing armors, part of that same training would be the casting spells under the duress of combat.
As one trains to become a SS part of the training would naturally include combat casting. During my toons training, it was part of the RP for her development and as a SS trainer she has RP'd it for those under her tutelage. Even for those journalling and RPing on their own to get the class (due to lack of GMs for cdqs), I would think it would be part of their development for the class.
I don't think this would overpower or unbalance the class. It would just removes the -4 concentration penalty to casting in the middle of a fight. They still have to add concentration points on their own when they level.
If it is approved, my recommendation would be to give it at Lvl 1 or 2 of the class.