2013 Regulations
The only article about the flexibility i found is:
3.17.6
The uppermost aerofoil element lying behind the rear wheel centre line may deflect no more
than 5mm horizontally when a 500N load is applied horizontally. The load will be applied
950mm above the reference plane at three separate points which lie on the car centre line
and 190mm either side of it. The loads will be applied in a rearward direction using a suitable
25mm wide adapter which must be supplied by the relevant team.
That article tells, that 5mm of flex is accepted by the fia during the test on the element itself. But the actuator that opens the DRS doesn't operate in strict horizontal direction. So it's the same as with the front wings, if you have a clever design that flexes more in the real race situation under the force of the actuator than in the FIA test it is legal until FIA changes the test.
And if you have to demonstrate during scrutineering that the gap between the two aero elements is legal, you simply could operate the actuator with less power/force than in the race.
Dear FIA, if you read this, please pm me for a redesign of the Technical Regulations to avoid finger nose shapes for 2016! :-)