You could just make a condition that is invoked on the character when the character chooses to change stance, example: The character chooses Offensive stance then a condition called "Offense" could be invoked on the character. The "Offense" condition doubles the character's attack. You could also be able to do this for Defensive and Mobile stance.

What I do is this - The command inflicts a status effect or alters the character's stats through a common event. Kick out to another common event that checks for status effects at the end of each player's turn. If the player is effected by a "stance" effect,  the character also gets another status effect added that doubles their speed (and if need be, inflicts a half speed on everyone else, friend and foe). That way, their next turn cycles much faster, so they only seem to lose "half" a turn by activating the skill. Every now and then, the enemy will still be able to slide a turn in between stance effects, but that seems fair. The only catch is that you have to make sure to cancel the speed effect on the character, and the slow effect on everyone else at the start of the character's next turn.

From your description, it might be better to manipulate the character's stats through common events. If you want to change critical hit chances but you don't want to subvert the dbs and calculate them on your own, you could use duplicate weapons, unless your game has so many weapons that this would just not be practical. Take a reading of the party's stats before combat though, so you have some variables to roll back to.

I did a simple tutorial on partially bypassing the dbs a while back, it might have some ideas you could cannibalize.

http://gamingw.net/forums/index.php?topic=81336.msg1585315#msg1585315