I wouldn't give much for your chances, although I have seen it done in the past. To a large extent it depends on the time of day and the mood of those involved.
The route does partially lie in Dutch airspace, however it is delegated to Brussel ACC - they therefore decide what can and can't be done. There is a very good reason for the one direction
As you already know the route you want to fly gets rejected; if you ask when airborne the first controller may approve it, and pass the request down the line but then discover that the next sector won't and you have to navigate back to the original, filed route.
There is a very god reason for the uni-directional airway. A considerable amount of the inbounds to London TMA route along it and all have to descend. The complexity of the airspace requires them to cross certain points at certain levels and by going against the flow
you are creating a much higher workload for all concerned. It is also the route followed by most of the EBBR departures to the west and they will be climbing all the way against you. I've seen 747s cross NIK at about 4000'.
Despite this, most controllers would do their best to accommodate your wishes, the easiest way to formulate it would be "Request direct NIK when available", on the frequency - would make you sound like a Ryanair pilot
