Are you allowed to contact a cushion behind the head string when shooting ball in hand from the kitchen?
E.g. with ball in hand in the kitchen, is It legal to kick off the side rail below the head string to strike a ball which is above the head string?
No you can not contact a cushion until the ball is outside of the kitchen. Doing so is a foul.
When a player has the cue ball in hand behind the head string (in the kitchen), he must drive the cue ball to a point across the head string before it contacts either a cushion, an object ball, or returns to the kitchen.
Failure to do so is a foul if a referee is presiding over a match. If no referee, the opponent has the option to call it either a foul or to require the offending player to replay the shot again with the balls restored to their positions prior to the shot (and with no foul penalty imposed).
Source: BCA "General Rules of Pocket Billiards"