If a box trigger is possible, then yes. I don't know if such a thing exists though (though it should, given that there's a Story function that looks inside the bounds of a trigger)
You might be able to accomplish something similiar by placing an item in the middle of the cell, and then using
CharacterGetDistanceToItem(CHARACTER Character, ITEM Item, OUT REAL Dist)
to check the player's distance from it. If they're outside the cell, the distance should be too big I would think.
INIT
DB_JailDistance(1,REAL:5);
KB Section
IF
CharacterSawCharacter(Guard, Player)
AND
CharacterGetDistanceToItem(Character_Player1, ITEM:Brazier, _CharDist)
AND
DB_JailDistance(1, _CheckDist)
AND
RealMax(_CharDist, _CheckDist, _MaxDist)
AND
DB_JailDistance(1, _MaxDist)
THEN
Stuff
