They can be quite sticky about entering at the Schengen side of the departure hall (hall 1) if you are travelling to a non-Schengen destination, they meaning the Schiphol security staff checking the boarding cards.
Sometimes you get away with saying that the queue at Non-Schengen is too long and they oblige.
But beware, the queue on the airside for going from Schengen to non-Schengen or vice versa can also be long!
Additional disadvantage is that on the Schengen side there is still a handluggage security check before you get to airside.
So travelling to BHX and entering at Schengen side and then going to non-Schengen will mean you have two checks, before entering airside and at the gate.

Another tip is for non-Schengen flights to use the immigration facilities in Departure hall 3 instead of 2. Unless there are many (Martinair) charters, this side is usually less busy than hall 2.

But back to the original topic: I have given up on the KL non-Schengen lounges. They are just too full too often.
I prefer to use the Globe Ground lounge with Priority Pass.
This is a great lounge, small but cosy and friendly and great mixed nuts!